docs: update .env.example for mongodb connection string

- Changes the example DATABASE_URL to the standard MongoDB format.

Author: fulleni

.env.example

@@ -2,4 +2,4 @@
 # Copy this file to .env and fill in your actual configuration values.
 # The .env file is ignored by Git and should NOT be committed.
 
-DATABASE_URL="postgres://user:password@localhost:5432/db_name"
+DATABASE_URL="mongodb://user:password@localhost:27017/ht_api_db"