README: Fix Config.FormatDSN url

Author: julienschmidt

README.md

@@ -93,7 +93,7 @@ This has the same effect as an empty DSN string:
 
 ```
 
-Alternatively, [Config.FormatDSN](https://godoc.org/github.com/go-sql-driver/mysql#FormatDSN) can be used to create a DSN string by filling a struct.
+Alternatively, [Config.FormatDSN](https://godoc.org/github.com/go-sql-driver/mysql#Config.FormatDSN) can be used to create a DSN string by filling a struct.
 
 #### Password
 Passwords can consist of any character. Escaping is **not** necessary.