Fix typo in SetValue validation exception message (#18197)

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -200,7 +200,7 @@ namespace Avalonia.PropertyStore
         {
             if (property.ValidateValue?.Invoke(value) == false)
             {
-                throw new ArgumentException($"{value} is not a valid value for '{property.Name}.");
+                throw new ArgumentException($"{value} is not a valid value for '{property.Name}'.");
             }
 
             if (priority != BindingPriority.LocalValue)