Fix Elm highlighting

Elm grammar repository contains several YAML files with a source.elm scope
We need to restrict the path to the Syntaxes directory
to make sure we select the appropriate YAML file

grammars.yml

@@ -20,7 +20,7 @@ vendor/grammars/ColdFusion:
 - text.html.cfm
 vendor/grammars/Docker.tmbundle:
 - source.dockerfile
-vendor/grammars/Elm:
+vendor/grammars/Elm/Syntaxes:
 - source.elm
 - text.html.mediawiki.elm-build-output
 - text.html.mediawiki.elm-documentation