[SPARK-3946] gitignore in /python includes wrong directory

Modified to ignore not the docs/ directory, but only the docs/_build/ which is the output directory of sphinx build.

Author: Masayoshi TSUZUKI <[email protected]>

Closes apache#2796 from tsudukim/feature/SPARK-3946 and squashes the following commits:

2bea6a9 [Masayoshi TSUZUKI] [SPARK-3946] gitignore in /python includes wrong directory

Author: tsudukim

Diff for: python/.gitignore

@@ -1,5 +1,5 @@
 *.pyc
-docs/
+docs/_build/
 pyspark.egg-info
 build/
 dist/