Documentation/CodingStyle: use the proper tag for verbatim font

On Sphinx/ReST notation, ``foo`` means that foo will be will be
marked as inline literal, effectively making it to be presented
as a monospaced font.

As we want this document to be parsed by Sphinx, instead of using
"foo", use ``foo`` for the names that are literal, because it is an
usual typographic convention to use monospaced fonts for functions
and language commands on documents, and we're following such
convention on the other ReST books.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>