typo with apt_po.pot in D-I

[Asho Yeh <ashoyeh@pchome.com.tw>]

Hello.

The apt_po.pot of D-I level 4 had a typo:

191:"  -o=? Set an arbitary configuration option, eg -o dir::cache=/tmp\n"

The "arbitary" may be a typo? I think it should be "arbitrary".

Best Regards
Asho Yeh