cmd/go/internal/modload: fix truncated error message from goModDirtyError

The 'go mod tidy' hint was truncated due to a typo in CL 293689,
and that particular case was not covered by any existing test.

Updates #36460
Updates #40775

Change-Id: Ib6fa872a9dfdafc4e9a112e8add2ff5aecd2dbd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/310089


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>