Fix for Observable attribute

Fix for #70

Author: sergey-tihon

Diff for: src/ProvidedTypes.fs

@@ -134,7 +134,7 @@ module internal Misc =
 #else
         { new CustomAttributeData() with 
 #endif
-                member __.Constructor =  typeof<System.ObsoleteAttribute>.GetConstructors() |> Array.find (fun x -> x.GetParameters().Length = 1)
+                member __.Constructor =  typeof<System.ObsoleteAttribute>.GetConstructors() |> Array.find (fun x -> x.GetParameters().Length = 2)
                 member __.ConstructorArguments = upcast [|CustomAttributeTypedArgument(typeof<string>, message) ; CustomAttributeTypedArgument(typeof<bool>, isError)  |]
                 member __.NamedArguments = upcast [| |] }