gw-gravity-forms-rounding.php: Update plugin installation instructions URL

Author: spivurno

gravity-forms/gw-gravity-forms-rounding.php

@@ -2,5 +2,5 @@
 /**
  * This snippet is now available as a free plugin with Spellbook. ✨
  * Instructions for installing this plugin can be found here:
- * https://gravitywiz.com/path-to-free-plugin-article/
+ * https://gravitywiz.com/gravity-forms-rounder/
  */