cmd/go: remove references to 1.13 in 'go help modules'

In "Module support" section, there were two mentions of "Go 1.13",
assuming that's the latest version. Rather than update these to 1.14,
this CL changes those to "The go command".

Also, a minor change in wording for finding go.mod files.

Change-Id: Id194be9405b540f221464814e71c361a22cc0f55
Reviewed-on: https://go-review.googlesource.com/c/go/+/214140


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>