Fix license string

Author: christophfroehlich

CONTRIBUTING.md

@@ -54,7 +54,9 @@ As this project, by default, uses the default GitHub issue labels
 
 
 ## Licensing
-Any contribution that you make to this repository will be under the Apache 2 License, as dictated by that [license]:
+Any contribution that you make to this repository will
+be under the Apache 2 License, as dictated by that
+[license](http://www.apache.org/licenses/LICENSE-2.0.html):
 
 ~~~
 5. Submission of Contributions. Unless You explicitly state otherwise,
@@ -69,4 +71,3 @@ Any contribution that you make to this repository will be under the Apache 2 Lic
 [issues]: https://github.com/ros-controls/kinematics_interface/issues
 [closed-issues]: https://github.com/ros-controls/kinematics_interface/issues?utf8=%E2%9C%93&q=is%3Aissue%20is%3Aclosed%20
 [help-wanted]: https://github.com/ros-controls/kinematics_interface/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22
-[license]: http://www.apache.org/licenses/LICENSE-2.0.html