Use string interpolation instead of calling Format

src/CalculatorUITestFramework/NumberPad.cs

@@ -77,7 +77,7 @@ namespace CalculatorUITestFramework
                         this.NegateButton.Click();
                         break;
                     default:
-                        throw (new ArgumentException(string.Format("{0} is not valid", digit)));
+                        throw (new ArgumentException($"{digit} is not valid"));
                 }
             }
         }