CONTRIBUTING.rst: Update CONTRIBUTING doc

Update link to compiling and building instructions to point to the
GitHub repo at the "obsproject" org URL. Replace link to Mantis with
link to GitHub Issues for our bug tracker to mirror the contents in
README.rst.

CONTRIBUTING.rst

@@ -5,10 +5,10 @@ Quick Links for Contributing
 ----------------------------
 
  - Compiling and building OBS Studio:
-   https://github.com/jp9000/obs-studio/wiki/Install-Instructions
+   https://github.com/obsproject/obs-studio/wiki/Install-Instructions
 
- - Our bug tracker (linked to forum accounts):
-   https://obsproject.com/mantis/
+ - Our bug tracker:
+   https://github.com/obsproject/obs-studio/issues
 
  - Discord Server: https://obsproject.com/discord