Rename HOWTORELEASE to RELEASING to follow pytest

Author: nicoddemus

HOWTORELEASE.rst renamed to RELEASING.rst

@@ -7,7 +7,7 @@ Release Procedure
 
    This will create the branch ready to be pushed.
 
-#. Open a PR targeting ``master``.
+#. Open a PR targeting ``main``.
 
 #. All tests must pass and the PR must be approved by at least another maintainer.
 
@@ -20,4 +20,4 @@ Release Procedure
 
 #. Make sure it is `available on PyPI <https://pypi.org/project/pluggy>`_.
 
-#. Merge the PR into ``master``, either manually or using GitHub's web interface.
+#. Merge the PR into ``main``, either manually or using GitHub's web interface.