update m5.md

Author: gslloyd

_milestones/m5.md

@@ -207,7 +207,7 @@ The display shows several control and status items.
       - on: continuous, useful in focusing the optics
       - shot: 200 ms pulse when trigger is pressed
     - Channel (CH): transmit channel, 0-9
-    - Volume (VOL): sound volume 0-100
+    - Volume (VOL): sound volume 0-100 in increments of 10
     - Threshold factor (THRESH): hit sensitivity of the detector in
       powers of 2
   - Status panel: