Add missing docs link in ReadingLevel Vale rule

doc/.vale/gitlab/ReadingLevel.yml

@@ -6,6 +6,7 @@
 # For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
 extends: readability
 message: "Grade level (%s) is high. To lower the score, use shorter sentences and words."
+link: https://docs.gitlab.com/ee/development/documentation/testing.html#vale-readability-score
 level: suggestion
 grade: 8
 metrics: