Update use-csharp-driver.txt

The original text suggests that you have to specify the replica set name in the connection string. Doing that you will get the error "Invalid key value pair".
Testcase:
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"