Update index-modules.asciidoc

Removed the experimental label from `index.code`

Closes #16644

docs/reference/index-modules.asciidoc

@@ -70,7 +70,7 @@ Checking shards may take a lot of time on large indices.
 
 [[index-codec]] `index.codec`::
 
-    experimental[] The +default+ value compresses stored data with LZ4
+    The +default+ value compresses stored data with LZ4
     compression, but this can be set to +best_compression+
     which uses https://en.wikipedia.org/wiki/DEFLATE[DEFLATE] for a higher
     compression ratio, at the expense of slower stored fields performance.