all: fix typos of possessive its

all: fix typos of possessive its

(I noticed the one mistake in hashtriemap.go and figured I'd clean up
others.)

Change-Id: I4ade424b400056f161bc6c9c2838ba1f96b1f6bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/615675
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>