Fix broken contributing URL link
We will always have a need for developers to help us improve Sponge. There is no such thing as a perfect project and things can always be improved. If you are a developer and are interested in helping then please do not hesitate. Just make sure you follow our guidelines.
-Please check our [Contribution Guidelines in the Sponge Documentation](https://docs.spongepowered.org/en/contributing/guidelines.html) for more information.
+Please check our [Contribution Guidelines in the Sponge Documentation](https://docs.spongepowered.org/stable/en/contributing/guidelines.html) for more information.
