Wrap YJIT guard clause in parentheses

Author: Bodacious

railties/lib/rails/generators/rails/app/templates/config/initializers/enable_yjit.rb.tt

@@ -4,7 +4,7 @@
 # If you are deploying to a memory constrained environment
 # you may want to delete this file, but otherwise it's free
 # performance.
-if defined? RubyVM::YJIT.enable
+if defined?(RubyVM::YJIT.enable)
   Rails.application.config.after_initialize do
     RubyVM::YJIT.enable
   end