Doc: fix default role usage (except in unittest mock docs)
diff --git a/Doc/howto/pyporting.rst b/Doc/howto/pyporting.rst
index e1a4609..483dcae 100644
--- a/Doc/howto/pyporting.rst
+++ b/Doc/howto/pyporting.rst
@@ -535,7 +535,7 @@
'''''''''''''''''''''''''''''''''''''''''''''
With Python 2, when ``map`` was given more than one input sequence it would pad
-the shorter sequences with `None` values, returning a sequence as long as the
+the shorter sequences with ``None`` values, returning a sequence as long as the
longest input sequence.
With Python 3, if the input sequences to ``map`` are of unequal length, ``map``