Documentation/kprobes: add s390 to list of supported architectures

s390 supports kprobes since 2006 but is missing in the list of
architectures that support kprobes.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>