cmd/go: fix "go help build -o" docs

The docs refer to "the last two paragraphs", but in fact should refer to
the first two of the previous three paragraphs. Moved up the out of place
paragraph.

Updates #14295

Change-Id: I066da7a665bc6754d246782b941af214a385017a
Reviewed-on: https://go-review.googlesource.com/c/go/+/190839


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>