Correct API docs for Span.lemma_, Vocab.to_bytes and Vectors.__init__

[schorfma]

Description

This pull request combines three small changes to the API docs to fix errors, I have stumbled upon while carefully reading on the API details.

Types of change

This correction for the API docs addresses:

1. Wrong code example for the property Span.lemma_ (First reported in #13405, Addressed in commit b334a7a)
2. Wrong return type (Vocab instead of bytes) for the method Vocab.to_bytes (Addressed in commit ce88925)
3. Slightly incorrect wording (Sequence instead of Iterable) in constructor method Vectors.__init__ (Addressed in commit d9c368d)

Checklist

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