Enclose the branch name in quotes for match with the RuboCop core

Author: koic

docs/antora.yml

@@ -2,6 +2,6 @@ name: rubocop-rails
 title: RuboCop Rails
 # We always provide version without patch here (e.g. 1.1),
 # as patch versions should not appear in the docs.
-version: master
+version: 'master'
 nav:
   - modules/ROOT/nav.adoc

tasks/cut_release.rake

@@ -39,7 +39,7 @@ namespace :cut_release do
 
     File.open('docs/antora.yml', 'w') do |f|
       f << antora_metadata.sub(
-        'version: master',
+        "version: 'master'",
         "version: '#{version_sans_patch(new_version)}'"
       )
     end