Replace deprecated link for ndjson (#2840)

* replace ndjson.org link

* replace ndjson link

Author: gmourier

learn/core_concepts/documents.mdx

@@ -110,7 +110,7 @@ In the above example:
 
 #### NDJSON
 
-NDJSON or jsonlines objects consist of individual lines where each individual line is valid JSON text and each line is delimited with a newline character. Any [rules that apply to formatting NDJSON](http://ndjson.org/) also apply to Meilisearch documents.
+NDJSON or jsonlines objects consist of individual lines where each individual line is valid JSON text and each line is delimited with a newline character. Any [rules that apply to formatting NDJSON](https://github.com/ndjson/ndjson-spec) also apply to Meilisearch documents.
 
 Meilisearch will only accept NDJSON documents when it receives the `application/x-ndjson` content-type header.