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

[cholmcc]

AliceO2/Utilities/rANS/benchmarks/bench_ransTPC.cxx

Line 381 in efe84cb

writer.Flush();

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.