fix link

Author: joao-r-reis

README.md

@@ -35,7 +35,7 @@ Installation
 
     go get github.com/apache/cassandra-gocql-driver/v2
 
-**Note:** Version `2.0.0` introduces breaking changes. See the [upgrade guide](UPGRADE_GUIDE.md) for upgrade instructions from `1.x`.
+**Note:** Version `2.0.0` introduces breaking changes. See the [upgrade guide](https://github.com/apache/cassandra-gocql-driver/blob/trunk/UPGRADE_GUIDE.md) for upgrade instructions from `1.x`.
 
 
 Features