extensions: libxt_bpf: clarify --bytecode argument

Mart Frauenlob suggested a change to explain the --bytecode
better. I have added some reference to the example bytecode
in the format that this argument accepts.

Reported-by: Mart Frauenlob <mart.frauenlob@chello.at>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/extensions/libxt_bpf.man b/extensions/libxt_bpf.man
index 4120a23..5b1d042 100644
--- a/extensions/libxt_bpf.man
+++ b/extensions/libxt_bpf.man
@@ -2,7 +2,7 @@
 is the format generated by the \fBnfbpf_compile\fP utility.
 .TP
 \fB\-\-bytecode\fP \fIcode\fP
-Pass the code in backtick format as argument.
+Pass the BPF byte code format (described in the example below).
 .PP
 The code format is similar to the output of the tcpdump -ddd command: one line
 that stores the number of instructions, followed by one line for each