Update Haraka help for plugin run order (#3405)

Fixed GetHooks function to return the array of plugin names.

This doesn't completely resolve the issue of Haraka help displaying
plugin run order. The Haraka installation still needs to install
Plugins.md into the docs directory.

bin/haraka

@@ -228,7 +228,7 @@ const plugin_doc = [
 
 
 function getHooks () {   // see haraka/Haraka#3306
-    fs.readFileSync('docs/Plugins.md').toString()
+    return fs.readFileSync('docs/Plugins.md').toString()
         .split('## Available Hooks')[1]    // discard everything before this string
         .split('### rcpt')[0]              // discard everything after this string
         .match(/\*\s(\S+)/gm)              // grab the first word of lines starting with '* '