Fix syntax error in stimulus_with_node.rb (#134)

Author: euxx

lib/install/stimulus_with_node.rb

@@ -15,7 +15,7 @@
 end
 
 say "Install Stimulus"
-if Rails.root.join("bun.config.js")).exist?
+if (Rails.root.join("bun.config.js")).exist?
   run "bun add @hotwired/stimulus"
 else
   run "yarn add @hotwired/stimulus"