Fix typo in MycroftSkill documentation

[PFischbeck]

Description

This fixes a link in the documentation of MycroftSkill, namely a double https://.

How to test

Rebuild the documentation and check the link in the mycroft.MycroftSkill documentation.

Contributor license agreement signed?

CLA [ yes ] (Whether you have signed a CLA - Contributor Licensing Agreement

[devs-mycroft]

Hello, @PFischbeck, thank you for helping with the Mycroft project! We welcome everyone
into the community and greatly appreciate your help as we work to build an AI
for Everyone.

To protect yourself, the project, and users of Mycroft technologies we require
a Contributor Licensing Agreement (CLA) before accepting any code
contribution. This agreement makes it crystal clear that along with your
code you are offering a license to use it within the confines of this project.
You retain ownership of the code, this is just a license.

Please visit https://mycroft.ai/cla to initiate this one-time signing. Thank
you!

[krisgesling]

Thanks Philipp, nice spotting!

Have sent the CLA