extra_tcp_stacks: Fix a few common typos

TCP_BBR:

• Fix a typo introducted in 1b90dfa5d2b0, which was reported by tuexen@

TCP_RACK:

• Correct two sysctl descriptions: s/corret/correct/

tcp_bbr(4): Also fix s/measurment/measurement/ in the man page

MFC after: 1 week