Update missing shakapacker spelling change

Author: ahangarha

lib/generators/react/component_generator.rb

@@ -258,7 +258,7 @@ def detect_template_extension
           "js.jsx.coffee"
         elsif options[:ts]
           "js.jsx.tsx"
-        elsif options[:es6] || webpacker?
+        elsif options[:es6] || shakapacker?
           "es6.jsx"
         else
           "js.jsx"