Validate 'ml.preview_datafeed' API

[sethmlarson]

Got an error for having an array of TDocument as a response body:

> [email protected] generate-schema /home/sethmlarson/elasticsearch-specification/compiler
> ts-node index.ts

Error: Children should be PropertySignature or PropertyDeclaration but is TypeReference instead
At: /home/sethmlarson/elasticsearch-specification/specification/ml/preview_datafeed/MlPreviewDatafeedResponse.ts:21:8

19  
20  export class Response<TDocument> {
21    body: TDocument[]
22  }
23  

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! [email protected] generate-schema: `ts-node index.ts`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the [email protected] generate-schema script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/sethmlarson/.npm/_logs/2021-07-29T19_21_45_101Z-debug.log
Makefile:38: recipe for target 'spec-generate' failed
make: *** [spec-generate] Error 1

Closes #377

[delvedor]

LGTM