docs/bpf: Fix a typo in verifier.rst

In verifier.rst, there is a typo in section 'Register parentage chains'.
Caller saved registers are r0-r5, callee saved registers are r6-r9.

Here by context it means callee saved registers rather than caller saved
registers. This may confuse users.
Signed-off-by: Yiming Xiang <kxiang@umich.edu>
Link: https://lore.kernel.org/r/20240829031712.198489-1-kxiang@umich.eduSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/verifier.rst

@@ -418,7 +418,7 @@ The rules for correspondence between registers / stack slots are as follows:
   linked to the registers and stack slots of the parent state with the same
   indices.
 
-* For the outer stack frames, only caller saved registers (r6-r9) and stack
+* For the outer stack frames, only callee saved registers (r6-r9) and stack
   slots are linked to the registers and stack slots of the parent state with the
   same indices.