Fix empty line in README. Fix contributing URL.

Signed-off-by: Chris Sanders <zidane@spongepowered.org>

.github/CONTRIBUTING.md

@@ -2,4 +2,4 @@
 
 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.

README.md

@@ -7,7 +7,7 @@ The SpongeAPI implementation targeting vanilla Minecraft and 3rd party platforms
 * [Source]
 * [Issues]
 * [Documentation]
-* [Discord]: `#sponge`
+* [Discord] `#sponge`
 
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