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>

Author: Yonghong Song

Documentation/bpf/verifier.rst

@@ -562,7 +562,7 @@ works::
   * ``checkpoint[0].r1`` is marked as read;
 
 * At instruction #5 exit is reached and ``checkpoint[0]`` can now be processed
-  by ``clean_live_states()``. After this processing ``checkpoint[0].r0`` has a
+  by ``clean_live_states()``. After this processing ``checkpoint[0].r1`` has a
   read mark and all other registers and stack slots are marked as ``NOT_INIT``
   or ``STACK_INVALID``