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>

Documentation/kprobes.txt

@@ -300,6 +300,7 @@ architectures:
 - arm
 - ppc
 - mips
+- s390
 
 3. Configuring Kprobes