fix code snippet (#7226)

fix incorrect code snippet

proposals/primary-constructors.md

@@ -24,7 +24,7 @@ public class C(bool b, int i, string s) : B(b) // b passed to base constructor
         get => s;
         set => s = value ?? throw new NullArgumentException(nameof(X));
     }
-    public C(string s) : this(0, s) { } // must call this(...)
+    public C(string s) : this(true, 0, s) { } // must call this(...)
 }
 ```