Replace `writer.Flush()` with `stream.Flush()`

[cholmcc]

https://github.com/AliceO2Group/AliceO2/blob/efe84cb78f0e72da74104dfb09d09edd9e7403b7/Utilities/rANS/benchmarks/bench_ransTPC.cxx#L381

rapidjson::Writer::Flush was introduced 15th of March, 2017, without updating the version number (last version update is from 25th of August, 2016 from 1.0.2 to 1.1.0). It is therefore not possible to detect at build time if Writer::Flush is available.

Note that rapidjson::Writer::Flush simply calls rapidjson::OStreamWrapper::Flush.

The authors of rapidjson should really get their act together and bump the version when they change the API.