cmd/go: fix unnecessary plural

"expanded imports paths" should read "expanded import paths." Run
mkalldocs.sh to pick up other changes which were not committed to
alldocs.go.

Change-Id: Iaa61e022d65f9464e8ff93a92cfba27dadf679cf
Reviewed-on: https://go-review.googlesource.com/127157


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>