nwid -> join

nwid -> join

Missed in the previous commit to the installer and wifi driver manuals:
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Use ifconfig(8)'s "join" command by default

Its adoption went quite well, so install "join" rather than the old "nwid"
in new hostname.if(5) files and follow this trend in our wifi manuals.

OK deraadt sthen
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Spotted/diff from From Raf Czlonka < rczlonka AT gmail DOT com>, thanks!

Tweak the example sentence while here.
Feedback jmc sthen