Correct PluginManager.add first parameter.

`PluginManager.add` accepts both plugin instances and plugin prototypes. The next version of TypeScript will support unions, at which time the type should be updated to `Phaser.Plugin|typeof Phaser.Plugin`. Until then we have to settle for `any` type.

Author: Bilge

typescript/phaser.d.ts

@@ -3161,7 +3161,7 @@ declare module Phaser {
         game: Phaser.Game;
         plugins: Phaser.Plugin[];
 
-        add(plugin: Phaser.Plugin, ...parameter: any[]): Phaser.Plugin;
+        add(plugin: any, ...parameter: any[]): Phaser.Plugin;
         destroy(): void;
         postRender(): void;
         postUpdate(): void;