doc: fix makeinfo errors and one warning

commit 4afa50b added few lines that are syntactically incorrect
with leading plus sign.

Cc: Denis Ovsienko <infrastation@yandex.ru>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/doc/ipv6.texi b/doc/ipv6.texi
index b6cc437..2482c1c 100644
--- a/doc/ipv6.texi
+++ b/doc/ipv6.texi
@@ -136,8 +136,8 @@
 Default: 0
 @end deffn
 
-+@deffn {Interface Command} {ipv6 nd home-agent-lifetime <0-65520>} {}
-+@deffnx {Interface Command} {no ipv6 nd home-agent-lifetime [<0-65520>]} {}
+@deffn {Interface Command} {ipv6 nd home-agent-lifetime <0-65520>} {}
+@deffnx {Interface Command} {no ipv6 nd home-agent-lifetime [<0-65520>]} {}
 The value to be placed in Home Agent Option, when Home Agent config flag is set, 
 which indicates to hosts Home Agent Lifetime. The default value of 0 means to
 place the current Router Lifetime value.