git primer: Fix two errors

Recommend pushing HEAD:main in the vendor branch docs, since that's
always going to be right.

Stop recommending people set push.default to 'freebsd'. It's an
enumerated value that's not a branch name. Most likely correct value
here is 'upstream'. However, there's enough people with asymetrical
setups that I'm just removing the setup rather than describe all the
special cases.

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