Update upgrade.asciidoc

Upgrade request needs pretty and human for the demonstrated output.

Closes #9313

docs/reference/indices/upgrade.asciidoc

@@ -36,7 +36,7 @@ you want to upgrade at the same time.
 
 [source,sh]
 --------------------------------------------------
-curl 'http://localhost:9200/twitter/_upgrade?human'
+curl 'http://localhost:9200/twitter/_upgrade?pretty&human'
 --------------------------------------------------
 
 [source,js]