gulp: fix completion function name

Fixes #6620

plugins/gulp/gulp.plugin.zsh

@@ -19,11 +19,11 @@
 # Grabs all available tasks from the `gulpfile.js`
 # in the current directory.
 #
-function $$gulp_completion {
+function _gulp_completion {
     compls=$(gulp --tasks-simple 2>/dev/null)
 
     completions=(${=compls})
     compadd -- $completions
 }
 
-compdef $$gulp_completion gulp
+compdef _gulp_completion gulp