Update GuiService Documentation to Fix Mismatched Reference (#1223)

Fixes mistake where `GuiService.SelectionOrder` was referenced instead
of `GuiObject.SelectionOrder`

## Changes

`GuiService.SelectionOrder` obviously doesn't exist, i am assuming that
it was originally supposed to say `GuiObject.SelectionOrder`.
This request fixes this issue

When i came across this, i was decently confused by this at first then
after a couple of minutes i realized it was a mistake which was a little
annoying...

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Co-authored-by: IgnisRBX <[email protected]>

Author: SeloTheGreat

content/en-us/reference/engine/classes/GuiService.yaml

@@ -713,7 +713,7 @@ methods:
       a descendant of PlayerGui, the engine searches all available selectable,
       visible and on-screen GuiObjects that are descendants of selectionParent
       and sets the `Class.GuiService.SelectedObject` to the GuiObject with the
-      smallest `Class.GuiService.SelectionOrder`.
+      smallest `Class.GuiObject.SelectionOrder`.
     code_samples:
       - GuiService-PreferredTransparency
     parameters: