add working query pipeline example code to elasticsearch-document-store.md

[annthurium]

fixes https://github.com/deepset-ai/haystack-integrations/issues/98

test strategy:

• [x] start up Elasticsearch instance
• [x] run code locally

output:

{'retriever': {'documents': [Document(id=6383dc3ed51fc90c2e45704853a7ad9b14168f4f262ac7a0b65e02c465d0bb1c, content: 'Historic Areas of Istanbul
With its strategic location on the Bosphorus peninsula between the Balkan...', meta: {'file_path': 'filename.txt', 'source_id': 'bb4b4bbec446c13f103bd0ec1777cbb424ae6d2bef355b17a5e299e97e9d4f55'}, score: 0.82134724, embedding: vector of size 1024)]}}