Fix OpenAPI heading level (#3179)

Author: guimachiavelli

reference/api/overview.mdx

@@ -9,7 +9,7 @@ This reference describes the general behavior of Meilisearch's RESTful API.
 
 If you are new to Meilisearch, check out the [getting started](/learn/self_hosted/getting_started_with_self_hosted_meilisearch).
 
-### OpenAPI
+## OpenAPI
 
 You can get the [Meilisearch OpenAPI specifications here](https://github.com/meilisearch/open-api/blob/main/open-api.json).