Fix slack greeting docs link

Author: Relequestual

slack/greeting

@@ -10,7 +10,7 @@ This server includes the specification authors, many implementers, and a large n
 
 :mag: If you're looking for an implementation of JSON Schema, you should check out the [implementations page](https://json-schema.org/implementations).
 
-:book: If you're looking to learn the basics of JSON Schema, do check out our [learning resources](https://json-schema.org/learn).
+:book: If you're looking to learn the basics of JSON Schema, do check out our [docs and learning resources](https://json-schema.org/overview/what-is-jsonschema).
 
 :question: If you want to ask questions or discuss JSON Schema, the #general channel is the right place.