Update CONTRIBUTING docs example link

[LorenzHenk]

Actually not a support request, but a note to help people in the future.

The link you mention here resolves to an old PR. Since that PR was merged, the docs system has changed a bit. Files, that were changed in the PR, do not exist any more.

Please update this link to a newer PR, so new contributors aren't confused.

Example PRs you could use:

• [docs] Add virtualized List example #15149
• [docs] Add a customization example with ToggleButton #17401
• [TextField] Add demo for color prop #18220
• [docs] Add ToggleButton demo for not accepting null value #19582 (disclamer: that one is by me 😁)

[oliviertassinari]

Right, thanks for noticing. Happy to use your Pull Request :)