Fix incorrect example in pipeline documentation

In our refdocs for pipeline definitions: (https://mongodb.github.io/mongo-csharp-driver/2.11/reference/driver/definitions/#pipelines)

We have the following pipeline definition in the docs

PipelineDefinition pipeline = new BsonDocument[] 

{

    new BsonDocument { { "$match", new BsonDocument("x", 1) } },

    new BsonDocument { { "$sort", new BsonDocument("y", 1) } }

};

 
I believe it should be:

PipelineDefinition<BsonDocument, BsonDocument> pipeline = new BsonDocument[] 

{

    new BsonDocument { { "$match", new BsonDocument("x", 1) } },

    new BsonDocument { { "$sort", new BsonDocument("y", 1) } }

};

Additionally, as rstam pointed out, this line is also incorrect:

> It is implicitly convertible from a List<BsonDocument>, a BsonDocument, a List<IPipelineStageDefinition> , and a IPipelineStageDefinition[].

It should state:
> It is implicitly convertible from a List<BsonDocument>, a BsonDocument[], a List<IPipelineStageDefinition> , and a IPipelineStageDefinition[].