cmd/cue: fix up get go docs around "enum" types
A legacy of pre-# definitions.
Change-Id: Ie341b5884672562bd525b64081faa40c5c379945
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9901
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>