Update branch names in release process (#444)

Co-authored-by: Pradyun Gedam <pradyunsg@users.noreply.github.com>
Co-authored-by: Brett Cannon <brett@python.org>

docs/development/release-process.rst

@@ -1,7 +1,7 @@
 Release Process
 ===============
 
-#. Checkout the current ``master`` branch.
+#. Checkout the current ``main`` branch.
 #. Install the latest ``nox``::
 
     $ pip install nox
@@ -20,6 +20,6 @@ Release Process
 
    Access needed for making the release are:
 
-   - PyPI maintainer (or owner) access to `packaging`
-   - push directly to the `master` branch on the source repository
+   - PyPI maintainer (or owner) access to ``packaging``
+   - push directly to the ``main`` branch on the source repository
    - push tags directly to the source repository