[int13] Fix typo in debug message

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/int13.c

 	 * to do so reliably.
 	 */
 
-	DBGC ( int13, "INT13 drive %02x unregsitered\n", int13->drive );
+	DBGC ( int13, "INT13 drive %02x unregistered\n", int13->drive );
 
 	/* Unhook INT 13 vector if no more drives */
 	if ( list_empty ( &int13s ) )