tc: man: SO_PRIORITY is described in socket documentation, not tc one

fix up reference: there is no tc(7) man page.
Signed-off-by: Florian Westphal <fw@strlen.de>

man/man8/tc-prio.8

@@ -50,7 +50,7 @@ be enqueued.
 From userspace
 A process with sufficient privileges can encode the destination class
 directly with SO_PRIORITY, see
-.BR tc(7).
+.BR socket(7).
 .TP 
 with a tc filter
 A tc filter attached to the root qdisc can point traffic directly to a class