This PR adds a test for an untested case of `Span.get_lca_matrix`, and fixes a bug for that scenario, which I introduced in [this PR](https://github.com/explosion/spaCy/pull/3089) (sorry!).
## Description
The previous implementation of get_lca_matrix was failing for the case `doc[j:k].get_lca_matrix()` where `j > 0`. A test has been added for this case and the bug has been fixed.
### Types of change
Bug fix
## Checklist
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.