docs: Fixing broken HTML

A couple of broken </code> tags were messing up the formatting
for the entire page (see b/134696600 , http://screen/aThdcpoDvJo.png )

Staged the fixed doc to:

http://go/dac-stage/reference/android/app/Activity

(The live version of this doc has already been fixed -- I fixed
the HTML by hand in cl/251937759 -- but we need to fix the Java
comment, or the file will revert when we publish beta 5.)

Bug: 134696600
Test: make ds-docs (& staged HTML output)
Exempt-From-Owner-Approval: Docs-only fix
Change-Id: I7e6d11ca358d747cdc6d04741d31c7acd85aa080