cmd/go/internal/modfetch: fix comment that mentions no-longer returned error
cmd/go/internal/modfetch: fix comment that mentions no-longer returned error

In c9211577eb77df9c51f0565f1da7d20ff91d59df @bcmills removed the returned error from
`Lookup`. However, the function docstring still mentions that this can return an error.

So this corrects the docs.

Change-Id: Idca74a200dfdb024e3d7ff6c439c70632cfec11a
GitHub-Last-Rev: 62eaacf70c3886d8aba25b79212c61485a89fde0
GitHub-Pull-Request: golang/go#45822
Reviewed-on: https://go-review.googlesource.com/c/go/+/314572
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go