pgsql: Update RELEASE_CHANGES' example of branch name format.

Update RELEASE_CHANGES' example of branch name format.

We're planning to put an underscore before the major version number in
branch names for v10 and later. Make sure the recipe in RELEASE_CHANGES
reflects that.

In passing, add a reminder to consider doing pgindent right before
the branch.

Discussion: /messages/by-id/E1dAkjZ-0003MG-0U@gemulon.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/655727d93bbaf2569281eea07e38b1955bb627b7

Modified Files
--------------
src/tools/RELEASE_CHANGES | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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