Trivial Typo Fix (#617)

Author: johnzhou721

content/contributing/sprint-guide/contents.lr

@@ -156,7 +156,7 @@ Set up a development environment
 ---------------------------------
 
 Once you've found an issue, you'll need to set up your development environment
-so you can work on a fix. Each BeeWare project has it's own contribution guide:
+so you can work on a fix. Each BeeWare project has its own contribution guide:
 
 * `Briefcase <https://briefcase.readthedocs.io/en/latest/how-to/contribute/index.html>`__
 * `Toga <https://toga.readthedocs.io/en/latest/how-to/contribute/index.html>`__