[SPARK-13350][DOCS] Config doc updated to state that PYSPARK_PYTHON's default is "python2.7"

Author: Christopher C. Aycock <chris@chrisaycock.com>

Closes #11239 from chrisaycock/master.

docs/configuration.md

@@ -1655,7 +1655,7 @@ The following variables can be set in `spark-env.sh`:
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     <td><code>PYSPARK_PYTHON</code></td>
-    <td>Python binary executable to use for PySpark in both driver and workers (default is <code>python</code>).</td>
+    <td>Python binary executable to use for PySpark in both driver and workers (default is <code>python2.7</code> if available, otherwise <code>python</code>).</td>
   </tr>
   <tr>
     <td><code>PYSPARK_DRIVER_PYTHON</code></td>