docs: use code-blocks

Sphinx can syntax-highlight a block if we use the right syntax.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>

docs/submittingpatches.rst

@@ -383,7 +383,7 @@ Git tips
 -  ``git rebase -i ...`` is your friend. Don't be afraid to use it.
 -  Apply a fixup to commit FOO.
 
-   ::
+   .. code-block:: console
 
       git add ...
       git commit --fixup=FOO
@@ -391,20 +391,20 @@ Git tips
 
 -  Test for build breakage between patches e.g last 8 commits.
 
-   ::
+   .. code-block:: console
 
       git rebase -i --exec="ninja -C build/" HEAD~8
 
 -  Sets the default mailing address for your repo.
 
-   ::
+   .. code-block:: console
 
       git config --local sendemail.to mesa-dev@lists.freedesktop.org
 
 -  Add version to subject line of patch series in this case for the last
    8 commits before sending.
 
-   ::
+   .. code-block:: console
 
       git send-email --subject-prefix="PATCH v4" HEAD~8
       git send-email -v4 @~8 # shorter version, inherited from git format-patch