Fix broken link to Contributions.txt
Fix broken link to Contributions.txt

Contributions.txt moved to the root of the edk2 repository.

Signed-off-by: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>

tianocore/tianocore.github.io,tianocore/tianocore.github.io