doc: fix quoted string in libxt_DNETMAP manpage

doc: fix quoted string in libxt_DNETMAP manpage

In roff, lines beginning with a single quote are control lines.  In the
libxt_DNETMAP man-page there is a single-quoted string at the beginning
of a line, which troff tries and fails to interpret as a macro:

	troff: <standard input>:49: warning: macro 'S'' not defined

This means that the line is not output.

Replace the single quotes with the appropriate escape-sequences.

Fixes: v2.3~9 ("doc: spelling and grammar corrections to DNETMAP")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>