Minor grammar improvement in types.rst (GH-11308)

defines utility function -> defines utility functions

Reported in https://mail.python.org/pipermail/docs/2018-December/038693.html

Doc/library/types.rst

@@ -8,7 +8,7 @@
 
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-This module defines utility function to assist in dynamic creation of
+This module defines utility functions to assist in dynamic creation of
 new types.
 
 It also defines names for some object types that are used by the standard