Fix composer.json to properly map \Piwik\Plugins to plugins/

composer.json

@@ -26,7 +26,7 @@
     "autoload": {
         "psr-0": {
             "Piwik\\Core\\": "core/",
-            "Piwik\\Plugin\\": "plugins/"
+            "Piwik\\Plugins\\": "plugins/"
         }
     }
 }