Update cars-assemble tasks description (#2908)

This is to try make it clearer that the production rate should take the success rate into account.

Author: tomasnorre

exercises/concept/cars-assemble/.docs/instructions.md

@@ -13,7 +13,7 @@ You have two tasks.
 
 ## 1. Calculate the production rate per hour
 
-Implement the `CarsAssemble.productionRatePerHour()` method to calculate the assembly line's production rate per hour, taking into account its current assembly line's speed :
+Implement the `CarsAssemble.productionRatePerHour()` method to calculate the assembly line's production rate per hour, taking into account its current assembly line's speed and success rate:
 
 ```Java
 CarsAssemble.productionRatePerHour(6)