Update contributing.md with correct commands
Update contributing.md with correct commands

Fixes #6

PiperOrigin-RevId: 309420188
Change-Id: Id0345a549bc17e02c05502e0c41b30c5402740c3

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks