bpf, docs: Fix verifier references

Use normal RST file reference instead of linkage copied from the old filter.rst
document that does not actually work when using HTML output.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-2-hch@lst.de