document fixing error in test output

#473 (comment)

Author: mlinksva

spec/ci_ruby_version_spec.rb

@@ -14,7 +14,7 @@
 
   context "in #{ci_config_file} and pages ruby version" do
     it 'match' do
-      msg = "(#{ci_ruby_version}) != (#{pages_ruby_version})"
+      msg = "#{ci_ruby_version} != #{pages_ruby_version}; please add a commit bumping in #{ci_config_file}"
       expect(ci_ruby_version).to eql(pages_ruby_version), msg
     end
   end