doc/ref/spec.md: fix doc bug in Label production
doc/ref/spec.md: fix doc bug in Label production

Change-Id: I3743dd87e788f0eeea060dcabbf3351e75d7bd3e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6145
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue