Repairing the link to sample DAG (#355)

In 
> Users could either load the data with [a python script](https://github.com/lyft/amundsendatabuilder/blob/master/example/scripts/sample_data_loader.py) with the library or with an [Airflow DAG](https://github.com/lyft/amundsendatabuilder/blob/master/example/dags/sample_dag.py) importing the library.
the link to the DAG example was broken, as the folder and file it pointed to no longer exist. I replaced the url with the new corresponding folder, but I didn't point to a specific file as there are now 3 different samples provided as example. So the link points to the folder: https://github.com/lyft/amundsendatabuilder/tree/master/example/dags

README.md

@@ -16,7 +16,7 @@ It includes three microservices, one data ingestion library and one common libra
 - [amundsenmetadatalibrary](https://github.com/lyft/amundsenmetadatalibrary#amundsen-metadata-service): Metadata service, which leverages Neo4j or Apache Atlas as the persistent layer, to provide various metadata.
 - [amundsendatabuilder](https://github.com/lyft/amundsendatabuilder#amundsen-databuilder): Data ingestion library for building metadata graph and search index.
 Users could either load the data with [a python script](https://github.com/lyft/amundsendatabuilder/blob/master/example/scripts/sample_data_loader.py) with the library
-or with an [Airflow DAG](https://github.com/lyft/amundsendatabuilder/blob/master/example/dags/sample_dag.py) importing the library.
+or with an [Airflow DAG](https://github.com/lyft/amundsendatabuilder/tree/master/example/dags) importing the library.
 - [amundsencommon](https://github.com/lyft/amundsencommon): Amundsen Common library holds common codes among microservices in Amundsen.