Fix typo

Author: dandv

source/reference/operator/aggregation/replaceRoot.txt

@@ -62,7 +62,7 @@ documents:
    ])
 
 Then the following :pipeline:`$replaceRoot` operation fails because one
-of the document does not have the ``name`` field:
+of the documents does not have the ``name`` field:
 
 .. code-block:: javascript