Corrected links to Briefcase contribution guide. (#674)

Corrects the links to the Briefcase contribution guide, as reported in beeware/beeware#534

Author: freakboy3742

content/contributing/process/contents.lr

@@ -17,11 +17,11 @@ GitHub repository for the project.
 Most projects have a dedicated contribution guide with details specific
 to that project, or specific types of contribution. This documentation
 can be found on Read the Docs. For example, [Briefcase's
-documentation](https://briefcase.readthedocs.io) contains contribution
+documentation](https://briefcase.beeware.org) contains contribution
 guides for both
-[code](https://briefcase.readthedocs.io/en/stable/how-to/contribute-code.html)
+[code](https://briefcase.beeware.org/en/stable/how-to/contribute/code.html)
 and
-[documentation](https://briefcase.readthedocs.io/en/stable/how-to/contribute-docs.html).
+[documentation](https://briefcase.beeware.org/en/stable/how-to/contribute/docs.html).
 
 All submissions should abide by the [BeeWare Code of
 Conduct](/community/behavior/code-of-conduct/).