update release procedure to clarify version bumping for next release

Author: shimizukawa

checklist.rst

@@ -6,4 +6,4 @@ Procedure:
 2. create GitHub Release with new version tag, it will create a release on PyPI.
    tag MUST following semver. e.g.: ``2.3.1``
 3. check PyPI page: https://pypi.org/p/sphinx-intl
-4. bump version in ``CHANGES.rst`` then commit/push it onto GitHub
+4. prepareing for the next release: bump version in ``CHANGES.rst`` then commit/push it onto GitHub