diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 968c999..885e765 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -4292,7 +4292,7 @@
 
    .. 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.
 
 .. seealso::
    :class:`types.MappingProxyType` can be used to create a read-only view