Documentation/powerpc: Fix ptrace request names

The documented ptrace request names are currently wrong/incomplete.
Fix this to improve correctness and searchability.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230725005841.28854-2-bgray@linux.ibm.com