Update PLUGIN-GUIDE.md

whitespace in plugin example

PLUGIN-GUIDE.md

@@ -168,7 +168,6 @@ You can add support for AMD/CommonJS loaders to your Leaflet plugin by following
     // define a Common JS module that relies on 'leaflet'
     } else if (typeof exports === 'object') {
         module.exports = factory(require('leaflet'));
-    
     }
 
     // attach your plugin to the global 'L' variable