doc/ref: fix a go-ism in the spec
doc/ref: fix a go-ism in the spec

It seems like it used ':=' instead of ':' by mistake.

Change-Id: I6f1694b029b43bb417c10c44048bd3976d4bb867
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4920
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue