Update primary-constructors.md

proposals/primary-constructors.md

@@ -16,13 +16,15 @@ The ability of a class or struct in C# to have more than one constructor provide
 Primary constructors put the parameters of one constructor in scope for the whole class or struct to be used for initialization or directly as object state. The trade-off is that any other constructors must call through the primary constructor.
 
 ``` c#
+using System;
+
 public class C(bool b, int i, string s) : B(b) // b passed to base constructor
 {
     public int I { get; set; } = i; // i used for initialization
     public string S // s used directly in function members
     {
         get => s;
-        set => s = value ?? throw new NullArgumentException(nameof(X));
+        set => s = value ?? throw new ArgumentNullException(nameof(X));
     }
     public C(string s) : this(0, s) { } // must call this(...)
 }