selftests/powerpc/ptrace: Fix typo in pid_max search error

pid_max_addr() searches for the 'pid_max' symbol in /proc/kallsyms, and
prints an error if it cannot find it. The error message has a typo,
calling it pix_max.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230725005841.28854-4-bgray@linux.ibm.com