Correct grammar mistake in stdtypes.rst (GH-10481)

Author: andresdelfino

Doc/library/stdtypes.rst

@@ -4335,7 +4335,7 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
 
    .. versionchanged:: 3.7
       Dictionary order is guaranteed to be insertion order.  This behavior was
-      implementation detail of CPython from 3.6.
+      an implementation detail of CPython from 3.6.
 
    Dictionaries and dictionary views are reversible. ::