README: Fix Item persistene reference

[jlaur]

The member was renamed from history in #331.

I tested if history is still available for backwards compatibility, but it appears not:

var averageSpotPrice = items.Energi_Data_Service_Spot_Price.history.averageBetween(tomorrowPeakStart, tomorrowPeakEnd);

Result:

Failed to execute action: 1(Error: Failed to execute rule Test-persistence-00356c1b-7b21-4fc2-8789-9b804de7577f: TypeError: undefined has no such function "averageBetween": TypeError: undefined has no such function "averageBetween"

[florian-h05]

Thanks for the fix 👍