Update src/ReactiveUI.Validation/Collections/ValidationText.cs

Co-authored-by: Artyom V. Gorchakov <worldbeater-dev@yandex.ru>

Author: glennawatson

src/ReactiveUI.Validation/Collections/ValidationText.cs

@@ -243,7 +243,7 @@ public void Clear()
 
             if (ReferenceEquals(this, None))
             {
-                throw new InvalidOperationException("Adding to ValidationText.None is unsupported.");
+                throw new InvalidOperationException("Clearing ValidationText.None is unsupported.");
             }
 
             _texts = Array.Empty<string>();