go/types: fix type in Interface.Complete method

This doesn't appear to have caused problems (because we don't depend
on the sort order, it seems) but it's clearly incorrect.

Change-Id: Ib6eb0128a3c17997c7907a618f9ce102b32aaa98
Reviewed-on: https://go-review.googlesource.com/79497


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>