Promote the new OpenAPI video course in the README

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

README.markdown

@@ -191,11 +191,12 @@ guidelines](https://github.com/sourcemeta/.github/blob/main/CONTRIBUTING.md).
 Meaningful contributions to this repository can be taken into consideration
 towards a discounted (or even free) commercial license.
 
-> [!TIP]
-> Do you want to level up your JSON Schema skills? Check out
+> [!TIP] Do you want to level up your JSON Schema skills? Check out
 > [learnjsonschema.com](https://www.learnjsonschema.com), our growing JSON
-> Schema documentation website, and our O'Reilly book [Unifying Business, Data,
-> and Code: Designing Data Products with JSON
+> Schema documentation website, our [JSON Schema for
+> OpenAPI](https://www.sourcemeta.com/courses/jsonschema-for-openapi) video
+> course, and our O'Reilly book [Unifying Business, Data, and Code: Designing
+> Data Products with JSON
 > Schema](https://www.oreilly.com/library/view/unifying-business-data/9781098144999/).
 
 ## :email: Contact