git_changelog: use modern format for rel branch names in example
e.g., REL_12_STABLE
# To generate major release notes, use:
# git_changelog --master-only --brief --oldest-first --since='start-date'
# To find the appropriate start date, use:
-# git show --summary $(git merge-base REL9_5_STABLE master)
+# git show --summary $(git merge-base REL_12_STABLE master)
# where the branch to mention is the previously forked-off branch. This
# shows the last commit before that branch was made.
#
