Fix HTML error.

Author: treeform

src/fidget/htmlbackend.nim

@@ -421,7 +421,7 @@ proc drawStart() =
 
   canvas.style.display = "block"
   canvas.style.position = "absolute"
-  canvas.style.zIndex = "-1"
+  canvas.style.zIndex = -1
   canvas.style.left = cstring($scrollBox.x & "px")
   canvas.style.top = cstring($scrollBox.y & "px")
   canvas.style.width = cstring($width & "px")