**Apache Airflow version**: 2.0.2

**What happened**:

Tried to use the following markdown as a `doc_md` string passed to a DAG

```markdown
- Example
    -  Nested List
```

It was rendered in the web UI as a single list with no nesting or indentation.

**What you expected to happen**:

I expected the list to display as a nested list with visible indentation.

**How to reproduce it**:

Try and pass a DAG a `doc_md` string of the above nested list.
I think the bug will affect any markdown that relies on meaningful indentation (tabs or spaces)

tests/www/test_utils.py
from airflow.www.app import cached_app
        with cached_app(testing=True).test_request_context():
            html = str(utils.dag_link({'dag_id': '<a&1>', 'execution_date': datetime.now()}))
        assert '%3Ca%261%3E' in html
        assert '<a&1>' not in html
    def test_dag_link_when_dag_is_none(self):
        """Test that when there is no dag_id, dag_link does not contain hyperlink"""
        from airflow.www.app import cached_app
        with cached_app(testing=True).test_request_context():
            html = str(utils.dag_link({}))
        assert 'None' in html
        assert '<a href=' not in html
    def test_dag_run_link(self):
        from airflow.www.app import cached_app
        with cached_app(testing=True).test_request_context():
            html = str(
                utils.dag_run_link({'dag_id': '<a&1>', 'run_id': '<b2>', 'execution_date': datetime.now()})
            )
        assert '%3Ca%261%3E' in html
        assert '%3Cb2%3E' in html
        assert '<a&1>' not in html
        assert '<b2>' not in html
class TestAttrRenderer(unittest.TestCase):