doc: escape minus sign in manpages

groff formats '-' as a hyphen, and '\-' is needed for a minus.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/extensions/libxt_state.man b/extensions/libxt_state.man
index 8e943c3..b5e719a 100644
--- a/extensions/libxt_state.man
+++ b/extensions/libxt_state.man
@@ -1,7 +1,7 @@
 This module, when combined with connection tracking, allows access to
 the connection tracking state for this packet.
 .TP
-[\fB!\fP] \fB--state\fP \fIstate\fP
+[\fB!\fP] \fB\-\-state\fP \fIstate\fP
 Where state is a comma separated list of the connection states to
 match.  Possible states are
 .B INVALID