Update Doc/library/datetime.rst

fix punctuation in docs, as requested

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: jyalim

Doc/library/datetime.rst

@@ -2673,7 +2673,7 @@ differences between platforms in handling of unsupported format specifiers.
    ``%G``, ``%u`` and ``%V`` were added.
 
 .. versionadded:: 3.12
-   ``%:z`` was added for :meth:`~.datetime.strftime`
+   ``%:z`` was added for :meth:`~.datetime.strftime`.
 
 .. versionadded:: 3.15
    ``%:z`` and ``%F`` were added for :meth:`~.datetime.strptime`