doc: resynchronize markup in iptables,ip6tables.8.in

Change .br to .PP in the intro section and use consistent
paragraphing in later ones.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/iptables.8.in b/iptables.8.in
index b9d6383..7754c8c 100644
--- a/iptables.8.in
+++ b/iptables.8.in
@@ -56,12 +56,11 @@
 filter rules in the Linux kernel.  Several different tables
 may be defined.  Each table contains a number of built-in
 chains and may also contain user-defined chains.
-
+.PP
 Each chain is a list of rules which can match a set of packets.  Each
 rule specifies what to do with a packet that matches.  This is called
 a `target', which may be a jump to a user-defined chain in the same
 table.
-
 .SH TARGETS
 A firewall rule specifies criteria for a packet and a target.  If the
 packet does not match, the next rule in the chain is the examined; if
@@ -342,7 +341,7 @@
 and you can use the \fB-h\fP or \fB--help\fP
 options after the module has been specified to receive help specific
 to that module.
-
+.PP
 The following are included in the base package, and most of these can
 be preceded by a "\fB!\fP" to invert the sense of the match.
 .\" @MATCH@