Change "tweet" type to "_doc" (#28690)

Elasticsearch 6.x indices do not allow multiple types index. Instead, they use "_doc" as default if created internally (Elasticsearch), or "doc" default if sent by Logstash.

Author: quentin-legraverend

docs/reference/docs/get.asciidoc

@@ -3,7 +3,7 @@
 
 The get API allows to get a typed JSON document from the index based on
 its id. The following example gets a JSON document from an index called
-twitter, under a type called tweet, with id valued 0:
+twitter, under a type called _doc, with id valued 0:
 
 [source,js]
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