Misplaced Playground in Functional Callback Section

[mrchoke]

Hi,

I found an issue with the placement of the Playground in the documentation. The Playground demo3 is currently located in the Functional callback section, but it should be in the Config block.

Could you please review and correct the placement?

Thank you!

documentation/docs/essential/plugin.md

Line 174 in ec3349f

<Playground :elysia="demo3" />