Update use-csharp-driver.txt

The original text suggests that you can to specify the replica set name in the connection string. Doing that you will get the error "Invalid key value pair". The following connection contains valid replica set name and server names and throws that exception: "mongodb://Mongo1,Mongo2,Mongo3/?connect=set1". The same code with the following connection works well: "mongodb://Mongo1,Mongo2,Mongo3/?connect=replicaset"