README: Fix broken link to examples.

The gltext examples were removed in go-gl/example#39
because they were unmaintained. Still, it's better to link to last
available version than to have a 404 link.

Updates #21.

README.md

@@ -52,7 +52,7 @@ licenses for more information about the requirements.
 
 Refer to [go-gl/examples/gltext][ex] for usage examples.
 
-[ex]: https://github.com/go-gl/examples/tree/master/gltext
+[ex]: https://github.com/go-gl/examples/tree/64b743f99c4e9151c09563e9be3339441eb9296b/gltext
 
 
 ### License