Change misleading code in Roact.Change example (#196)

GetPropertyChangedSignal does not pass the new value of the property to the function and the Roact.Change handler doesn't add this functionality so the current example code is slightly misleading.

docs/api-reference.md

@@ -142,8 +142,8 @@ It's similar to `Roact.Event`:
 
 ```lua
 Roact.createElement("ScrollingFrame", {
-	[Roact.Change.CanvasPosition] = function(rbx, position)
-		print("ScrollingFrame scrolled to", position)
+	[Roact.Change.CanvasPosition] = function(rbx)
+		print("ScrollingFrame scrolled to", rbx.CanvasPosition)
 	end,
 })
 ```
@@ -395,4 +395,4 @@ end
 As with `setState`, you can set use the constant `Roact.None` to remove a field from the state.
 
 !!! note
-	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.
+	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.