README: Update reference to renamed example repo. (#81)

The old link still worked thanks to the GitHub redirect, but it's better to point
to the new repo directly.

Updates go-gl/example#58.
Follows #78.

README.md

@@ -44,7 +44,7 @@ A note about threading and goroutines. The bindings do not expose a mechanism to
 Examples
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-Examples illustrating how to use the bindings are available in the [examples](https://github.com/go-gl/examples) repo. There are examples for [OpenGL 4.1 core](https://github.com/go-gl/example/tree/master/gl41core-cube) and [OpenGL 2.1](https://github.com/go-gl/example/tree/master/gl21-cube).
+Examples illustrating how to use the bindings are available in the [example](https://github.com/go-gl/example) repo. There are examples for [OpenGL 4.1 core](https://github.com/go-gl/example/tree/master/gl41core-cube) and [OpenGL 2.1](https://github.com/go-gl/example/tree/master/gl21-cube).
 
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