go/types: update flag documentation for gotype command

The documentation refers to a non longer existing flag (-seq).
Remove those references.

Change-Id: I480b6259f9199b47761dc655a90911eabfe07427
Reviewed-on: https://go-review.googlesource.com/c/go/+/249738


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>