s390/watchdog: Fix module name in Kconfig help text

The help text for DIAG288_WATCHDOG in drivers/watchdog/Kconfig
still mentioned the misleading old module name vmwatchdog.
This patch changes that to the correct new name diag288_wdt.
Signed-off-by: Philipp Hachtmann <phacht@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/watchdog/Kconfig

@@ -1293,7 +1293,7 @@ config DIAG288_WATCHDOG
 	  both.
 
 	  To compile this driver as a module, choose M here. The module
-	  will be called vmwatchdog.
+	  will be called diag288_wdt.
 
 # SUPERH (sh + sh64) Architecture