Fix typo on engine.inc documentation (#730)

`register_think` first parameter is `Classname[]`, but, it's written `Touched` on its description.

plugins/include/engine.inc

@@ -117,7 +117,7 @@ native register_touch(const Touched[], const Toucher[], const function[]);
  *       blocking the think. To immediately block return PLUGIN_HANDLED_MAIN
  *       instead.
  *
- * @param Touched   Entity classname to hook
+ * @param Classname	Entity classname to hook
  * @param function	Name of callback function
  *
  * @return 	        Think forward id