docs/bpf: Fix an incorrect statement in verifier.rst

docs/bpf: Fix an incorrect statement in verifier.rst

In verifier.rst, I found an incorrect statement (maybe a typo) in section
'Liveness marks tracking'. Basically, the wrong register is attributed
to have a read mark. This may confuse the user.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240111052136.3440417-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>