Fix typo in Koji documentation
Fix typo in Koji documentation

The media_types key lives in build.extra.image, not build.extra.

Signed-off-by: Tim Waugh <fe049355006bf7fdd11c5e47ffe7a5032af30704@redhat.com>

maxamillion/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,vrutkovs/atomic-reactor,maxamillion/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor