docs: process: Add an example for creating a fixes tag

To make it a little clearer how to create a fixes tag,
add an example based on the preceeding gitconfig setup.

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20200710200115.21176-1-trix@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>