[MINOR][DOC] Fix style in examples across documentation

## What changes were proposed in this pull request?

This PR fixes the documentation as below:

  -  Python has 4 spaces and Java and Scala has 2 spaces (See https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide).

  - Avoid excessive parentheses and curly braces for anonymous functions. (See https://github.com/databricks/scala-style-guide#anonymous)

## How was this patch tested?

N/A

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #14593 from HyukjinKwon/minor-documentation.

1 files changed, 2 insertions, 2 deletions

diff --git a/docs/programming-guide.md b/docs/programming-guide.md
index f82832905e..40287d7702 100644
--- a/docs/programming-guide.md
+++ b/docs/programming-guide.md
@@ -1516,8 +1516,8 @@ data.map(x -> { accum.add(x); return f(x); });
 {% highlight python %}
 accum = sc.accumulator(0)
 def g(x):
-  accum.add(x)
-  return f(x)
+    accum.add(x)
+    return f(x)
 data.map(g)
 # Here, accum is still 0 because no actions have caused the `map` to be computed.
 {% endhighlight %}