Add a flush to test in _cat/indices.asciidoc

We test that sorting by `store.size` works but sometimes the
sizes aren't what we expect. At least in CI:
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-unix-compatibility/os=opensuse/101/console
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+5.x+multijob-unix-compatibility/os=centos/100/console

I haven't been able to reproduce it locally but adding a `_flush`
won't hurt and might make the inconsistency vanish.

docs/reference/cat/indices.asciidoc

@@ -67,6 +67,7 @@ GET /_cat/indices?v&s=store.size:desc
 --------------------------------------------------
 // CONSOLE
 // TEST[continued]
+// TEST[s/^/POST _flush\n/]
 
 Which looks like: