chore: fix docs URL in badge

[sk1p]

rtd.io is on longer (?) operational, according to the project page the correct domain is either rtfd.io or the full readthedocs.io. Also switch to https.

Purpose of PR?:

Fix broken badge.

Checklist:

• Bug fix. Fixes #
• New feature (Non-API breaking changes that adds functionality)
• PR Title follows the convention of <type>: <subject>
• Commit has unit tests

[ReimarBauer]

@sk1p Thanks, that you help us. \o/
Please can you send this us to stable, we do bug fixes there. That gets then merged into develop.

[sk1p]

@sk1p Thanks, that you help us. \o/
Please can you send this us to stable, we do bug fixes there. That gets then merged into develop.

Sure, I've changed the base branch of the pr to stable, hope that worked out.

[ReimarBauer]

@sk1p Thanks, that you help us. \o/
Please can you send this us to stable, we do bug fixes there. That gets then merged into develop.

Sure, I've changed the base branch of the pr to stable, hope that worked out.

Currently develop has no further changes than the release. squashing should be fine.

[ReimarBauer]

ah, that needs a rebase to stable, because the skyfield-data fix is only there.

[sk1p]

ah, that needs a rebase to stable, because the skyfield-data fix is only there.

Rebased.