GOROOT is not a function, so use "is" instead of "returns".
This was originally raised during review of golang.org/cl/131315.
Also add a missing period.
Change-Id: Ie28d38529889702ae5035d0f48d8122b6bee6819
Reviewed-on: https://go-review.googlesource.com/131316
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>