validate: give hint on how to regenerate outdated REFERENCE.md

Author: kenyon

lib/puppet-strings/tasks/validate.rb

@@ -34,7 +34,7 @@
       existing = File.read(filename)
 
       if generated != existing
-        warn "#{filename} is outdated"
+        warn "#{filename} is outdated; to regenerate: bundle exec rake strings:generate:reference"
         exit 1
       end
     end