update root folder link (#377)

UPGRADE.md

@@ -51,7 +51,7 @@ Upgrade from 2.1.9
 Upgrade from 2.0.5
 ----------------------
 
-* PHP [mongodb](https://php.net/manual/en/set.mongodb.php) extension is now used instead of [mongo](http://php.net/manual/en/book.mongo.php).
+* PHP [mongodb](https://php.net/manual/en/set.mongodb.php) extension is now used instead of [mongo](https://php.net/manual/en/book.mongo.php).
   Make sure you have 'mongodb' extension at your environment. Some features based on old driver may become unavailable.
   In particular: fields `Connection::mongoClient`, `Database::mongoDb` and `Collection::mongoCollection` are no longer exist.
   Old driver type classes such as `\MongoId`, `\MongoCode`, `\MongoDate` and so on, are no longer returned or