doc/ref/spec.md: a few fixes
doc/ref/spec.md: a few fixes

- boolean_lit -> bool_lit
- removed reference to top_lit
- added bottom_lit

Fixes #773
Fixes #777

Change-Id: I40e103d5fdd73f1ef99c89a10333b6abb5aad6d3
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8741
Reviewed-by: Paul Jolly <a027184a55211cd23e3f3094f1fdc728df5e0500@myitcv.org.uk>

cuelang/cue