[MINOR][DOCS] Replace non-breaking space to normal spaces that breaks rendering markdown

# What changes were proposed in this pull request?

It seems there are several non-breaking spaces were inserted into several `.md`s and they look breaking rendering markdown files.

These are different. For example, this can be checked via `python` as below:

```python
>>> " "
'\xc2\xa0'
>>> " "
' '
```

_Note that it seems this PR description automatically replaces non-breaking spaces into normal spaces. Please open a `vi` and copy and paste it into `python` to verify this (do not copy the characters here)._

I checked the output below in  Sapari and Chrome on Mac OS and, Internal Explorer on Windows 10.

**Before**

![2017-04-03 12 37 17](https://cloud.githubusercontent.com/assets/6477701/24594655/50aaba02-186a-11e7-80bb-d34b17a3398a.png)
![2017-04-03 12 36 57](https://cloud.githubusercontent.com/assets/6477701/24594654/50a855e6-186a-11e7-94e2-661e56544b0f.png)

**After**

![2017-04-03 12 36 46](https://cloud.githubusercontent.com/assets/6477701/24594657/53c2545c-186a-11e7-9a73-00529afbfd75.png)
![2017-04-03 12 36 31](https://cloud.githubusercontent.com/assets/6477701/24594658/53c286c0-186a-11e7-99c9-e66b1f510fe7.png)

## How was this patch tested?

Manually checking.

These instances were found via

```
grep --include=*.scala --include=*.python --include=*.java --include=*.r --include=*.R --include=*.md --include=*.r -r -I " " .
```

in Mac OS.

It seems there are several instances more as below:

```
./docs/sql-programming-guide.md:        │   ├── ...
./docs/sql-programming-guide.md:        │   │
./docs/sql-programming-guide.md:        │   ├── country=US
./docs/sql-programming-guide.md:        │   │   └── data.parquet
./docs/sql-programming-guide.md:        │   ├── country=CN
./docs/sql-programming-guide.md:        │   │   └── data.parquet
./docs/sql-programming-guide.md:        │   └── ...
./docs/sql-programming-guide.md:            ├── ...
./docs/sql-programming-guide.md:            │
./docs/sql-programming-guide.md:            ├── country=US
./docs/sql-programming-guide.md:            │   └── data.parquet
./docs/sql-programming-guide.md:            ├── country=CN
./docs/sql-programming-guide.md:            │   └── data.parquet
./docs/sql-programming-guide.md:            └── ...
./sql/core/src/test/README.md:│   ├── *.avdl                  # Testing Avro IDL(s)
./sql/core/src/test/README.md:│   └── *.avpr                  # !! NO TOUCH !! Protocol files generated from Avro IDL(s)
./sql/core/src/test/README.md:│   ├── gen-avro.sh             # Script used to generate Java code for Avro
./sql/core/src/test/README.md:│   └── gen-thrift.sh           # Script used to generate Java code for Thrift
```

These seems generated via `tree` command which inserts non-breaking spaces. They do not look causing any problem for rendering within code blocks and I did not fix it to reduce the overhead to manually replace it when it is overwritten via `tree` command in the future.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17517 from HyukjinKwon/non-breaking-space.

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index d0eca1ddea..1e521a7e7b 100644
--- a/README.md
+++ b/README.md
@@ -97,7 +97,7 @@ building for particular Hive and Hive Thriftserver distributions.
 Please refer to the [Configuration Guide](http://spark.apache.org/docs/latest/configuration.html)
 in the online documentation for an overview on how to configure Spark.
 
-## Contributing
+## Contributing
 
 Please review the [Contribution to Spark guide](http://spark.apache.org/contributing.html)
 for information on how to get started contributing to the project.