Fix broken link in aggregate reference docs

This page: https://mongodb.github.io/mongo-csharp-driver/2.11/reference/driver/definitions/#pipelines

Links to: https://github.com/mongodb/mongo-csharp-driver/blob/master/tests/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectionTranslatorTests_Group.cs

This file doesn't exist. Should the link point to https://github.com/mongodb/mongo-csharp-driver/blob/master/tests/MongoDB.Driver.Tests/Linq/Translators/AggregateGroupTranslatorTests.cs instead?