Fix nested string formatting in DoubleBox's return statement

Revised the return statement to correctly handle nested string formatting by ensuring the format string is constructed properly. This resolves potential formatting issues with dynamic values.

TBF/Boxes/DoubleBox.cs

@@ -76,7 +76,7 @@ namespace TBF.Boxes
 			}
 			else
 			{
-                return string.Format( $"{{0:{Format}}}", Val * Factor);
+                return string.Format(string.Format("{{0:{0}}}", Format), Val * Factor);
 			}
         }