Update RELEASING document

Tags should be GPG signed.

[ci skip]

RELEASING.md

@@ -4,7 +4,7 @@
 2. Update `NEWS.md` to reflect the changes since last release.
 3. Commit changes. There shouldn't be code changes, and thus CI doesn't need to
    run, you can then add `[ci skip]` to the commit message.
-4. Tag the release: `git tag vVERSION -a`. The tag message should contain the
+4. Tag the release: `git tag vVERSION -a -s`. The tag message should contain the
    appropriate `NEWS.md` subsection.
 5. Push changes: `git push --tags`
 6. Build and publish to rubygems: