[SPARK-12760][DOCS] invalid lambda expression in python example for …

…local vs cluster

srowen thanks for the PR at https://github.com/apache/spark/pull/10866! sorry it took me a while.

This is related to https://github.com/apache/spark/pull/10866, basically the assignment in the lambda expression in the python example is actually invalid

```
In [1]: data = [1, 2, 3, 4, 5]
In [2]: counter = 0
In [3]: rdd = sc.parallelize(data)
In [4]: rdd.foreach(lambda x: counter += x)
  File "<ipython-input-4-fcb86c182bad>", line 1
    rdd.foreach(lambda x: counter += x)
                                   ^
SyntaxError: invalid syntax
```

Author: Mortada Mehyar <mortada.mehyar@gmail.com>

Closes #10867 from mortada/doc_python_fix.

1 files changed, 5 insertions, 2 deletions

diff --git a/docs/programming-guide.md b/docs/programming-guide.md
index bad25e63e8..4d21d4320c 100644
--- a/docs/programming-guide.md
+++ b/docs/programming-guide.md
@@ -789,9 +789,12 @@ counter = 0
 rdd = sc.parallelize(data)
 
 # Wrong: Don't do this!!
-rdd.foreach(lambda x: counter += x)
+def increment_counter(x):
+    global counter
+    counter += x
+rdd.foreach(increment_counter)
 
-print("Counter value: " + counter)
+print("Counter value: ", counter)
 
 {% endhighlight %}
 </div>