Update ps3000a.py

Added missing comma at line 156

Author: utkuturker

picosdk/ps3000a.py

@@ -153,7 +153,7 @@ def _define_digital_port():
     "PS3000A_ABOVE_LOWER",
     "PS3000A_BELOW_LOWER",
     "PS3000A_RISING_LOWER",
-    "PS3000A_FALLING_LOWER"
+    "PS3000A_FALLING_LOWER",
     "PS3000A_POSITIVE_RUNT",
     "PS3000A_NEGATIVE_RUNT"
     ])