man: netsniff-ng: Fix a typo

Replace 'networkoing' by 'networking'.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
man: netsniff-ng: Fix typo of 'privileges'

Replace 'priviledges' by 'privileges'.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
man: netsniff-ng: Do some wordsmithing

Fix wording and phrasing in some of the usage examples.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>