filter.txt: update 'tools/net/' to 'tools/bpf/'

The tools are located at tootls/bpf/ instead of tools/net/.
Update the filter.txt doc.

Signed-off-by: Wang Sheng-Hui <shhuiw@foxmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed