[API] indices.upgrade's wait_for_completion param defaults to false

Documentation is correct but the API spec has it backwards.

Code: https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/rest/action/admin/indices/upgrade/RestUpgradeAction.java#L93
Docs: https://github.com/elasticsearch/elasticsearch/blob/master/docs/reference/indices/upgrade.asciidoc#request-parameters