Update Gruntfile.js

Added punctuation to make the comment consistent with other comments.

Gruntfile.js

@@ -55,7 +55,7 @@ module.exports = function(grunt) {
   grunt.registerTask('lint', ['eslint']);
 
   grunt.registerTask('delete-build-modules', function() {
-    // Use gulp here
+    // Use gulp here.
     spawnGulp(['react:clean'], null, this.async());
   });