documentation: update s390 header file paths

Update Documentation/s390/ files to reflect changed header files
locations.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/s390/Debugging390.txt

@@ -1402,7 +1402,7 @@ Syscalls are implemented on Linux for S390 by the Supervisor call instruction (S
 possibilities of these as the instruction is made up of a  0xA opcode & the second byte being
 the syscall number. They are traced using the simple command.
 TR SVC  <Optional value or range>
-the syscalls are defined in linux/include/asm-s390/unistd.h
+the syscalls are defined in linux/arch/s390/include/asm/unistd.h
 e.g. to trace all file opens just do
 TR SVC 5 ( as this is the syscall number of open )