Add missing markup (GH-17680)

"HH", "MM" and "ffffff" are enclosed with double back quotes, but "SS" is left being bare

Author: cocoatomo

Doc/library/datetime.rst

@@ -2514,7 +2514,7 @@ Notes:
       :meth:`utcoffset` is transformed into a string of the form
       ``±HHMM[SS[.ffffff]]``, where ``HH`` is a 2-digit string giving the number
       of UTC offset hours, ``MM`` is a 2-digit string giving the number of UTC
-      offset minutes, SS is a 2-digit string giving the number of UTC offset
+      offset minutes, ``SS`` is a 2-digit string giving the number of UTC offset
       seconds and ``ffffff`` is a 6-digit string giving the number of UTC
       offset microseconds. The ``ffffff`` part is omitted when the offset is a
       whole number of seconds and both the ``ffffff`` and the ``SS`` part is