**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)

airflow/www/utils.py
"""Returns a formatted string with HTML for given JSON serializable"""
    def json_(attr):
        f = attr.get(attr_name)
        serialized = json.dumps(f)
        return Markup('<nobr>{}</nobr>').format(serialized)  
    return json_
def dag_link(attr):
    """Generates a URL to the Graph view for a Dag."""
    dag_id = attr.get('dag_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.graph', dag_id=dag_id, execution_date=execution_date)
    return Markup('<a href="{}">{}</a>').format(url, dag_id) if dag_id else Markup('None')  
def dag_run_link(attr):
    """Generates a URL to the Graph view for a DagRun."""
    dag_id = attr.get('dag_id')
    run_id = attr.get('run_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.graph', dag_id=dag_id, run_id=run_id, execution_date=execution_date)
    return Markup('<a href="{url}">{run_id}</a>').format(url=url, run_id=run_id)  
def pygment_html_render(s, lexer=lexers.TextLexer):