updated (m)termvectors api specs to reflect #7530

#7530 introduced an option to specify `doc` instead of `id` this change was not included in the rest spec. 

mtermvectors listed id as part but its not a part of the url.

Will open a separate PR against 1.4 branch which does not have the `dfs` feature introduced here: https://github.com/elasticsearch/elasticsearch/pull/8144

@alexksikes please correct me if I misunderstood the new feature.
should be `_termvectors`