Correct typo in comment (#17)

Author: timneutkens

README.md

@@ -100,7 +100,7 @@ new FriendlyErrorsPlugin({
   },
   onErrors: function (severity, errors) {
     // You can listen to errors transformed and prioritized by the plugin
-    // sevrity can be 'error' or 'warning'
+    // severity can be 'error' or 'warning'
   },
   // should the console be cleared between each compilation?
   // default is true