Removed extra word "a"

Author: sharshi

source/core/2d.txt

@@ -73,7 +73,7 @@ Collation Option
 ``2d`` indexes only support simple binary comparison and do not support
 the :ref:`collation <collation>` option.
 
-To create a ``2d`` index on a a collection that has a non-simple
+To create a ``2d`` index on a collection that has a non-simple
 collation, you must explicitly specify ``{collation: {locale: "simple"}
 }`` when creating the index.