hello: printk KERN_ALERT -> KERN_DEBUG
hello: changed printk(KERN_DEBUG to pr_debug()

Changed the printk(KERN_DEBUG statements in hello.c to pr_debug() according
to recommended practices.

jeri@hudson:~/linux$ ./scripts/checkpatch.pl --file ~/ldd/hello/hello.c
WARNING: Prefer [subsystem eg: netdev]_dbg([subsystem]dev, ... then dev_dbg(dev, ... then pr_debug(...  to printk(KERN_DEBUG ...
+	printk(KERN_DEBUG "Hello, World\n");

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>