Fix a few link references (#420)

docs/tutorials/data-preview-with-superset.md

 # How to setup a preview client with Apache Superset
 
-In the previous [tutorial](docs/tutorials/index-postgres.md), we talked about how to index the table metadata
+In the previous [tutorial](./index-postgres.md), we talked about how to index the table metadata
 for a postgres database. In this tutorial, we will walk through how to configure data preview for this `films` table
 using Apache Superset.
 

docs/tutorials/index-postgres.md

 # How to index metadata for real life databases
 
-From previous [doc](docs/installation.md), we have indexed tables from a csv files. In real production cases, 
+From previous [doc](../installation.md), we have indexed tables from a csv files. In real production cases, 
 the table metadata is stored in data warehouses(e.g Hive, Postgres, Mysql, Snowflake, Bigquery etc.) which Amundsen has 
 the extractors for metadata extraction.