core-spec: Modify `.lhs` to `.hs`
Modify old filename .lhs
to .hs
in following files:
- docs/core-spec/README
- docs/core-spec/CoreLint.ott
- docs/core-spec/CoreSyn.ott
- docs/core-spec/core-spec.mng
- docs/core-spec/core-spec.pdf (generated PDF)
[ci skip]