Update prettier hook details

Per prettier/prettier#8937 the pre-commit hook has
been moved to https://github.com/pre-commit/mirrors-prettier. I have also
updated to the latest version in that repository.

.pre-commit-config.yaml

@@ -34,8 +34,8 @@ repos:
       - id: markdownlint
         args:
           - --config=.mdl_config.json
-  - repo: https://github.com/prettier/pre-commit
-    rev: v2.1.2
+  - repo: https://github.com/pre-commit/mirrors-prettier
+    rev: v2.2.1
     hooks:
       - id: prettier
   - repo: https://github.com/adrienverge/yamllint