Addied missing "=" in "IsBillable = false "

The "eqauls" operation was using just one =.

Author: HaikoHertes

articles/azure-monitor/logs/analyze-usage.md

@@ -220,7 +220,7 @@ let freeTables = dynamic([
 Usage 
 | where DataType !in (freeTables) 
 | where TimeGenerated > ago(30d) 
-| where IsBillable = false 
+| where IsBillable == false 
 | summarize MonthlyPotentialUnderbilledGB=sum(Quantity)/1000 by DataType
 ```