Merge branch 'docs-ci-cache-key-nesting' into 'master'

Fix nesting in cache key fallback entry

See merge request gitlab-org/gitlab!54282

doc/ci/yaml/README.md

@@ -2812,7 +2812,7 @@ URI-encoded `%2F`. A value made only of dots (`.`, `%2E`) is also forbidden.
 
 You can specify a [fallback cache key](#fallback-cache-key) to use if the specified `cache:key` is not found.
 
-#### Fallback cache key
+##### Fallback cache key
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1534) in GitLab Runner 13.4.