Fix broken url in CONTRIBUTING.md

[posquit0]

I found the broken url in CONTRIBUTING.md file!

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[posquit0]

I signed it

[posquit0]

/assign @chrislovecnm

[rdrgmnzs]

/area documentation
/ok-to-test
/lgtm

[mikesplain]

Thanks so much @posquit0!

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mikesplain, posquit0, rdrgmnzs

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