[S390] Fix register usage description.

Fix description of register usage as pointed out by Andreas Krebbel.
Since this document is completely outdated and would need a lot of
fixing, it might be worth considering to get rid of it...
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/s390/Debugging390.txt

@@ -480,7 +480,7 @@ r2       argument 0 / return value 0                call-clobbered
 r3       argument 1 / return value 1 (if long long) call-clobbered
 r4       argument 2                                 call-clobbered
 r5       argument 3                                 call-clobbered
-r6	 argument 5                                 saved
+r6	 argument 4				    saved
 r7       pointer-to arguments 5 to ...              saved      
 r8       this & that                                saved
 r9       this & that                                saved