Update GuiService Documentation to Fix Mismatched Reference

[SeloTheGreat]

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...

Checks

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[github-actions]

Hi @SeloTheGreat, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.