Fix remote name in "Migrating from GitHub fork"

[arichardson]

The new FreeBSD repository no longer contains the old master branch so we can't use freebsd/master in the rebase command. Update the instructions to use origin for the GitHub remote.

[bsdimp]

More likely, we need to talk about freebsd-legacy now and/or talk about what would have been in the repos. 'origin' here makes me uneasy due to the diversity of github origin names I've seen while helping people.

[arichardson]

More likely, we need to talk about freebsd-legacy now and/or talk about what would have been in the repos. 'origin' here makes me uneasy due to the diversity of github origin names I've seen while helping people.

Maybe we could suggest git remote rename <github-remote> freebsd-github-legacy and then use that name? I agree that origin is probably not ideal unless we expect people to continue to treat the github mirror as the default remote.