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_tcpmss.man b/extensions/libxt_tcpmss.man
index 01cdc3a..8ee715c 100644
--- a/extensions/libxt_tcpmss.man
+++ b/extensions/libxt_tcpmss.man
@@ -1,4 +1,4 @@
 This matches the TCP MSS (maximum segment size) field of the TCP header.  You can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.
 .TP
-[\fB!\fP] \fB--mss\fP \fIvalue\fP[\fB:\fP\fIvalue\fP]
+[\fB!\fP] \fB\-\-mss\fP \fIvalue\fP[\fB:\fP\fIvalue\fP]
 Match a given TCP MSS value or range.