**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
#
#
#
import json
import time
from urllib.parse import urlencode
import markdown
import sqlalchemy as sqla
from flask import Markup, Response, request, url_for
from flask_appbuilder.forms import FieldConverter
from flask_appbuilder.models.sqla import filters as fab_sqlafilters
from flask_appbuilder.models.sqla.interface import SQLAInterface
from pygments import highlight, lexers
from pygments.formatters import HtmlFormatter  
from airflow.utils import timezone
from airflow.utils.code_utils import get_python_source
from airflow.utils.json import AirflowJsonEncoder
from airflow.utils.state import State
from airflow.www.forms import DateTimeWithTimezoneField
from airflow.www.widgets import AirflowDateTimePickerWidget
def get_sensitive_variables_fields():