Fix link in CONTRIBUTING.md.
Fix link in CONTRIBUTING.md.

PiperOrigin-RevId: 194929266
Change-Id: Ib93d5a38c64b01b6851f76a9324b184aefddce9d

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor