Change maxCount to firstQuartileCount in plastics.json

[DainialPadraig]

The maxCount (rank 1) value in the plastics JSON file needs to be changed to firstQuartileCount with the value for the rank 3 country instead. This will make calculating the mission count more straight-forward, since the mission count can be calculated between the medianCount and the firstQuartileCount.

The potential downside of this change is that, either the data file will need to be modified again or logic will need to be added to choose a different upper bound for the rand() function (like the rank 2 count), if the first quartile value makes the game too easy.