[System] Change S.N.S.NetworkStream's param name on ctor

Changed to the name that is used in MS.NET:
http://msdn.microsoft.com/en-us/library/te7e60bx%28v=vs.110%29.aspx

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=16832
Merge pull request #841 from knocte/master

[System] Change S.N.S.NetworkStream's param name on ctor