doc/ref/spec.md: fix typos
doc/ref/spec.md: fix typos

This PR fixes 3 typos in the spec.

Closes #954
https://github.com/cuelang/cue/pull/954

GitOrigin-RevId: cbebd270d204885a44cd4c3dfb5c7688bc59a4a3
Change-Id: Id70a300a695390404297d44b32dd811f6fef7164
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9781
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue