Update RegExp syntax link to RE2

CI ref pattern matching now uses RE2 instead of Ruby.
See
https://about.gitlab.com/2019/04/01/security-release-gitlab-11-dot-9-dot-4-released/ for more information.

doc/ci/yaml/README.md

@@ -319,7 +319,7 @@ There are a few rules that apply to the usage of job policy:
 
 - `only` and `except` are inclusive. If both `only` and `except` are defined
    in a job specification, the ref is filtered by `only` and `except`.
-- `only` and `except` allow the use of regular expressions (using [Ruby regexp syntax](https://ruby-doc.org/core/Regexp.html)).
+- `only` and `except` allow the use of regular expressions ([supported regexp syntax](#supported-onlyexcept-regexp-syntax)).
 - `only` and `except` allow to specify a repository path to filter jobs for
    forks.