Fix typo in Session docstring (#886)

Author: kmorkos

mongo/session.go

@@ -100,7 +100,7 @@ func SessionFromContext(ctx context.Context) Session {
 // resources are properly cleaned up, context deadlines and cancellations will not be respected during this call. For a
 // usage example, see the Client.StartSession method documentation.
 //
-// ClusterTime, OperationTime, Client, and ID return the session's current operation time, the session's current cluster
+// ClusterTime, OperationTime, Client, and ID return the session's current cluster time, the session's current operation
 // time, the Client associated with the session, and the ID document associated with the session, respectively. The ID
 // document for a session is in the form {"id": <BSON binary value>}.
 //