docs: Typo fix ("performed an down" -> "performed a down")

Needed to make a docs fix to verify the (updated) "how to fix a reference
doc bug" instructions (go/fix-refdocs), so I picked this one. Very low
priority, so if merging would be a hassle we don't need to do it.

Bug: 324457037
Change-Id: I216238d8e0da9fafb1fa5a7531f839b6360b5081
Test: ABTD docs build go/abtd/run/L98000030001930492