time range [beginning of day] - [beginning of next day] is needed for getStats() with StatAPI to get 1 day of data

[zcernigoj]

Mention that to get 1 day of statistical data with getStats() when StatAPI is used, you need to set time range to [beginning of day] - [beginning of next day] instead of [beginning of day] - [end of day] like for getMap(), getTiles(), ...

sentinelhub-js/README.md

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## Getting basic statistics and histogram