ci/config: correct instruction for CI preferences

From e76eec3554 (ci: allow per-branch config for GitHub Actions,
2020-05-07), we started to allow contributors decide which branch
they want to build with GitHub Actions
by checking for a file named "ci/config/allow-ref".

In order to assist those contributors,
we provided a sample in "ci/config/allow-refs.sample",
and instructed them to drop the ".sample",
then commit that file to their repository.

We've misspelt the filename in that change.
Let's fix the spelling.

While we're at it, also instruct our contributors introduce that new
file to Git before commit, in case of they've never told Git before.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/config/allow-ref.sample

@@ -7,8 +7,9 @@
 # your repository:
 #
 #   git checkout -b ci-config
-#   cp allow-refs.sample allow-refs
-#   $EDITOR allow-refs
+#   cp allow-ref.sample allow-ref
+#   $EDITOR allow-ref
+#   git add allow-ref
 #   git commit -am "implement my ci preferences"
 #   git push
 #