doc: fix seg6_flowlabel path

This sysctl is not per interface; it's global per netns.

Fixes: 292ecd9 ("doc: move seg6_flowlabel to seg6-sysctl.rst")
Reported-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: NicolasDichtel

Documentation/networking/seg6-sysctl.rst

@@ -25,6 +25,9 @@ seg6_require_hmac - INTEGER
 
 	Default is 0.
 
+/proc/sys/net/ipv6/seg6_* variables:
+====================================
+
 seg6_flowlabel - INTEGER
 	Controls the behaviour of computing the flowlabel of outer
 	IPv6 header in case of SR T.encaps