Fix a bug in Sound.tone translation to beep

Author: ddemidov

ev3dev/core.py

@@ -2349,7 +2349,7 @@ def beep_args(frequency=None, duration=None, delay=None):
                 args = '-n '
                 if frequency is not None: args += '-f %s ' % frequency
                 if duration  is not None: args += '-l %s ' % duration
-                if delay     is not None: args += '-d %s ' % delay
+                if delay     is not None: args += '-D %s ' % delay
 
                 return args