bpf, docs: Add a missing colon in verifier.rst

Add a missing colon to fix the document style.

Fixes: 88691e9e ("bpf, docs: Split general purpose eBPF documentation out of filter.rst")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220228080416.1689327-1-wanjiabing@vivo.com