Update invalid links in README.md

[JeffResc]

Proposed change

This updates the links to the frontend development page in the README file.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

N/A

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[frenck]

The developers' website doesn't use .html extension anymore.

[JeffResc]

I used those to keep consistency as I saw many links using .html, my apologies, I’ll exclude that from now on

[frenck]

👍 Thanks for updating @JeffResc