[doc] po4a(7) man page: one more typo

[Fat-Zer]

one more typo:
doc/po4a.7.pod:751

once before editing the PO files to update them and once afterward to get a
completely updated translated document.

Reported on #22 before.

[mquinson]

I am playing with the idea of adding a --untypot parameter to the po4a script. This would help dealing with such situations in a better way. Until then, we'll have to fix it manually.