chore: correct the contributor link in CONTRIBUTING.md

[ulivz]

Summary

Now the clickable contributor link in .github/CONTRIBUTING.md is not correct that will go to 404.

• It's caused by the auto-completed wrong link: https://github.com/vuejs/vue/blob/dev/.github/graphs/contributors
• And the correct link is https://github.com/vuejs/vue/graphs/contributors

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

[defcc]

LGTM. Thanks @ulivz