fix lsp-extensions section header

Author: BenjaminBrienen

docs/book/src/contributing/lsp-extensions.md

@@ -653,7 +653,7 @@ Note that this functionality is intended primarily to inform the end user about
 In particular, it's valid for the client to completely ignore this extension.
 Clients are discouraged from but are allowed to use the `health` status to decide if it's worth sending a request to the server.
 
-### Controlling Flycheck
+## Controlling Flycheck
 
 The flycheck/checkOnSave feature can be controlled via notifications sent by the client to the server.