Adjust heading size in readme

README.md

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 # mautrix-whatsapp
 A Matrix-WhatsApp puppeting bridge based on [whatsmeow](https://github.com/tulir/whatsmeow).
 
-### Documentation
+## Documentation
 All setup and usage instructions are located on [docs.mau.fi]. Some quick links:
 
 [docs.mau.fi]: https://docs.mau.fi/bridges/go/whatsapp/index.html