s390/Documentation: improve sort command for trace buffer

When s390 traces with hex_ascii or sprintf view are
extracted and sorted, use the sort option -s (stable)
to avoid multiple lines with the same time stamp being
sorted using the rest of the line as secondary key.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/s390/s390dbf.txt

@@ -405,7 +405,7 @@ Example:
 
 > ls /sys/kernel/debug/s390dbf/dasd
 flush  hex_ascii  level pages raw
-> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort +1
+> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s
 00 00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....
 00 00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE
 00 00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....