Docs: Update docs/index_.asciidoc
Use `_doc` in the routing example instead of `tweet` to agree with the
text and line up with the other examples.
@@ -249,7 +249,7 @@ on a per-operation basis using the `routing` parameter. For example:
[source,js]
--------------------------------------------------
-POST twitter/tweet?routing=kimchy
+POST twitter/_doc?routing=kimchy
{
"user" : "kimchy",
"post_date" : "2009-11-15T14:12:12",
