doc: fix the PR template CONTRIBUTING.md link

[gibfahn]

Using ./CONTRIBUTING.md in the PR template resolves to
https://github.com/nodejs/citgm/compare/CONTRIBUTING.md. Use the full path
instead (as is done in Node.js core).

Note that the CONTRIBUTING.md file hasn't yet been added (the PR to add it is
#314).

Link to core PR template: https://raw.githubusercontent.com/nodejs/node/master/.github/PULL_REQUEST_TEMPLATE.md

Checklist

• documentation is changed or added
• contribution guidelines followed here
• commit message follows commit guidelines

[gibfahn]

I'd say it doesn't need to wait for #314 , there's already a link to CONTRIBUTING.md in the PULL_REQUEST template, this just fixes the link.

[gdams]

@gibfahn think this is good to land