docs: Added missing closing parenthesis in sample code

The code sample within the "Return a Result" section of the "Allowing
Other Apps to Start Your Activity" page was missing a closing
parenthesis at the end of the second line. This parenthesis should now
appear.

Bug: 26845664
Change-Id: Iec58b86889d6e8fa6ad5327678d5e6d63d34f85f