tweak the Description of the OnDemandPercentage param

clarify that the stack will use spot instances when the price is lower
than the standard OnDemand price.

Author: yob

templates/aws-stack.yml

@@ -221,7 +221,7 @@ Parameters:
     Default: 0
 
   OnDemandPercentage:
-    Description: Percentage of total instances that should launch as OnDemand. Reduce this to use some Spot Instances when they're available.
+    Description: Percentage of total instances that should launch as OnDemand. Default is 100% OnDemand - reduce this to use some Spot Instances when they're available and cheaper than the OnDemand price.
     Type: Number
     Default: 100
     MinValue: 0