s390/dis: use correct escape sequence for '%' character

The double escape character sequence introduced with commit
272fa59c ("s390/dis: Fix handling of format specifiers") is not
necessary anymore since commit 561e1030 ("s390/dis: Fix printing
of the register numbers").

Instead this now generates an extra '%' character:

lg      %%r1,160(%%r11)

So fix this and basically revert 272fa59c.

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