DOCSP-7911: Correction needed for shardKey description in Spark Connector docs (#36)

Author: steveren

source/configuration.txt

@@ -493,7 +493,10 @@ The following options for writing to MongoDB are available:
 
    * - ``shardKey``
 
-     - The write concern :ref:`wTimeout <wc-wtimeout>` value.
+     - The field by which to split the collection data. The field
+       should be indexed and contain unique values.
+
+       *Default*: ``_id``
 
    * - ``forceInsert``