git_changelog: use modern format for rel branch names in example
e.g., REL_12_STABLE
Branch------master
Details-------https://git.postgresql.org/pg/commitdiff/395a9a124877d3c41328fcfebcf0c68df86d9bfd
Modified Files--------------src/tools/git_changelog | 2 +-1 file changed, 1 insertion(+), 1 deletion(-)
