Fix ruby cop warning

Signed-off-by: Gavin Didrichsen <gavin.didrichsen@gmail.com>

lib/puppetlabs_spec_helper/puppetlabs_spec/puppet_internals.rb

@@ -39,7 +39,7 @@ def function_method(name, parts = {})
       # exposed hack.
       return nil unless Puppet::Parser::Functions.function(name)
 
-      scope.method("function_#{name}".to_sym)
+      scope.method(:"function_#{name}")
     end
     module_function :function_method
   end