Fix typo in MWAA getting started guide (#846)

## Description

This fixes a typo in the MWAA getting started guide. We instruct the
user to create a virtualenv using MWAA's startup script, but then the
execution config uses the virtual env mode which will re-create the venv
every task.

Instead, the user should let Cosmos use the default (local) execution
mode.

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## Related Issue(s)

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## Breaking Change?

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## Checklist

- [ ] I have made corresponding changes to the documentation (if
required)
- [ ] I have added tests that prove my fix is effective or that my
feature works

docs/getting_started/mwaa.rst

@@ -104,7 +104,6 @@ In your ``my_cosmos_dag.py`` file, import the ``DbtDag`` class from Cosmos and c
 
     execution_config = ExecutionConfig(
         dbt_executable_path=f"{os.environ['AIRFLOW_HOME']}/dbt_venv/bin/dbt",
-        execution_mode=ExecutionMode.VIRTUALENV,
     )
 
     my_cosmos_dag = DbtDag(