Update set.txt

Added a missing 's' to the word "update" to correct sentence grammar.

Author: silvermanj7

source/reference/operator/update/set.txt

@@ -104,7 +104,7 @@ Set Elements in Arrays
 .. include:: /includes/use-dot-notation.rst
 
 For the document matching the criteria ``_id`` equal to ``100``, the
-following operation update the value second element (array index of
+following operation updates the value second element (array index of
 ``1``) in the ``tags`` field and the ``rating`` field in the first
 element (array index of ``0``) of the ``ratings`` array.