Generalise example from README (#1311)

docs/getting_started/astro.rst

@@ -90,6 +90,11 @@ In your ``my_cosmos_dag.py`` file, import the ``DbtDag`` class from Cosmos and c
     from cosmos import DbtDag, ProjectConfig, ProfileConfig, ExecutionConfig
     from cosmos.profiles import PostgresUserPasswordProfileMapping
 
+    import os
+    from datetime import datetime
+
+    airflow_home = os.environ["AIRFLOW_HOME"]
+
     profile_config = ProfileConfig(
         profile_name="default",
         target_name="dev",
@@ -101,11 +106,11 @@ In your ``my_cosmos_dag.py`` file, import the ``DbtDag`` class from Cosmos and c
 
     my_cosmos_dag = DbtDag(
         project_config=ProjectConfig(
-            "/usr/local/airflow/dags/my_dbt_project",
+            f"{airflow_home}/dags/my_dbt_project",
         ),
         profile_config=profile_config,
         execution_config=ExecutionConfig(
-            dbt_executable_path=f"{os.environ['AIRFLOW_HOME']}/dbt_venv/bin/dbt",
+            dbt_executable_path=f"{airflow_home}/dbt_venv/bin/dbt",
         ),
         # normal dag parameters
         schedule_interval="@daily",