Correct OS names to match official style guides (#628)

Author: johnzhou721

content/project/briefcase/contents.lr

@@ -24,7 +24,7 @@ application. You can package projects for:
 * Android
 * Web
 
-Support for AppleTV, watchOS, and wearOS is planned.
+Support for Apple TV, watchOS, and Wear OS is planned.
 
 If you want to see Briefcase in action, try the `BeeWare tutorial
 <https://beeware.readthedocs.io>`__. That tutorial walks you through the