ArangoSerilogSink example need to be updated

[bbm-design]

ArangoSerilogSink doesn't match the example anymore

batchPostingLimit: 50, TimeSpan.FromSeconds(2)

aren't part of the ctor anymore, and when removed

you get : cannot convert from 'Core.Arango.Serilog.ArangoSerilogSink' to 'Serilog.Core.ILogEventSink'