Fix typo in register_token_generator docstring

[2tunnels]

In generate_token method we are getting default generator with: func = self._token_generators.get('default').

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Other, please describe: docstring update.

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

(If no, please delete the above question and this text message.)

---

• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.