README.md: Update link to BPF docs in kernel repository

The BPF documentation has evolved quite a bit since 2016. File
filter.txt is not the best link to provide here, we can instead point to
the directory Documentation/bpf/ which contains more resources.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

README.md

@@ -700,8 +700,7 @@ details.
 * Cilium project documentation about BPF: [_BPF and XDP Reference
   Guide_](http://docs.cilium.io/en/latest/bpf/)
 
-* Kernel documentation about BPF: [Documentation/networking/filter.txt
-  file](https://www.kernel.org/doc/Documentation/networking/filter.txt)
+* [Kernel documentation about BPF](https://docs.kernel.org/bpf/)
 
 * [_Dive into BPF: a list of reading
   material_](https://qmonnet.github.io/whirl-offload/2016/09/01/dive-into-bpf),