**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
"""Generates a URL to the Graph view for a TaskInstance."""
    dag_id = attr.get('dag_id')
    task_id = attr.get('task_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.task', dag_id=dag_id, task_id=task_id, execution_date=execution_date.isoformat())
    url_root = url_for(
        'Airflow.graph', dag_id=dag_id, root=task_id, execution_date=execution_date.isoformat()
    )
    return Markup(  
        """
        <span style="white-space: nowrap;">
        <a href="{url}">{task_id}</a>
        <a href="{url_root}" title="Filter on this task and upstream">
        <span class="material-icons" style="margin-left:0;"
            aria-hidden="true">filter_alt</span>
        </a>
        </span>
        """
    ).format(url=url, task_id=task_id, url_root=url_root)
def state_token(state):
    """Returns a formatted string with HTML for a given State"""
    color = State.color(state)
    fg_color = State.color_fg(state)
    return Markup(  
        """
        <span class="label" style="color:{fg_color}; background-color:{color};"
            title="Current State: {state}">{state}</span>
        """
    ).format(color=color, state=state, fg_color=fg_color)
def state_f(attr):