moretypes: fixed minor typo

Updates golang/go#14040.

Fixes `structwhen` to become `struct when`.

Change-Id: I2cc4f9bded5d4ec4922d8daf49dbb6738210c096
Reviewed-on: https://go-review.googlesource.com/20028
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed