[DOCS] Change `_routing` to `routing` in mget API docs (#76214) (#76304)

The request body parameter is `routing`, not `_routing`.

Co-authored-by: Daniel Imfeld <daniel@imfeld.dev>

docs/reference/docs/multi-get.asciidoc

@@ -100,7 +100,7 @@ document:
 The index that contains the document.
 Required if no index is specified in the request URI.
 
-`_routing`::
+`routing`::
 (Optional, string) The key for the primary shard the document resides on.
 Required if routing is used during indexing.