Update TextChatCommand.yaml to specify folder requirement

[Vexot]

Changes

Clarified that TextChatCommands must be parented under the folder named TextChatCommands within TextChatService in order to work, instead of just parented to the service.

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[github-actions]

Hi @Vexot, 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.

[IgnisRBX]

I just tested this using the example in https://create.roblox.com/docs/en-us/chat/examples/custom-text-chat-commands, and it works with the command parented to just TextChatService.