cmd/go: reference BuildID field from list's -export docs
In https://golang.org/cl/263542 we added BuildID to the Package struct
in the docs for "go list", correctly pointing out that it's only set
when -export is used.
Further down, the doc details the -export flag on its own.
It already mentioned the Export field, and we forgot to add a mention to
BuildID as well. Do that.
Change-Id: I5838a8900edae8012fe333937d86baea3066c5f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/392114
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>