docs: Fix phrasing for BroadcastReceiver.PendingResult

Phrasing was a little garbled, I felt safe cleaning it up but please
correct me if I got it wrong!

NO_TEST , fixes a doc bug (no code edits).

Test: [go/abtd docs build]
Fixes: 289051151
Change-Id: Ibe22b10335a100c5d25d31bebc02dca439dca5f6