Default number of shards is now 1 instead of 5 (#43573)

As specified in the [Breaking changes for 7.X](https://www.elastic.co/guide/en/elasticsearch/reference/7.1/breaking-changes-7.0.html#breaking_70_indices_changes), the default number of shards for an index is now `1` instead of `5`.

docs/reference/index-modules.asciidoc

@@ -36,7 +36,7 @@ specific index module:
 
 `index.number_of_shards`::
 
-    The number of primary shards that an index should have.  Defaults to 5.
+    The number of primary shards that an index should have.  Defaults to 1.
     This setting can only be set at index creation time.  It cannot be
     changed on a closed index. Note: the number of shards are limited to `1024` per
     index. This limitation is a safety limit to prevent accidental creation of indices