Link to section on CSS Transitions instead of Mozilla page

[Pi-Cla]

Description

In the Animations section of the documentation, I had expected that the link labeled "CSS Transitions" would jump me to the CSS Transitions section. Instead it redirected me to a Mozilla webpage which was unexpected.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Yup, that's a good change. This MDN link is in that section anyway.