Document the `Optimizer.last_score` property

[danieldk]

Description

Document the Optimizer.last_score property. This property was added in Thinc v9, but was not documented.

Types of change

Checklist

• [x] I confirm that I have the right to submit this contribution under the project's MIT license.
• [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.