**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
"""
    FAB does not know how to handle columns with leading underscores because
    they are not supported by WTForm. This hack will remove the leading
    '_' from the key to lookup the column names.
    """
    def __init__(self, obj, session=None):
        super().__init__(obj, session=session)
        def clean_column_names():
            if self.list_properties:
                self.list_properties = {k.lstrip('_'): v for k, v in self.list_properties.items()}
            if self.list_columns:
                self.list_columns = {k.lstrip('_'): v for k, v in self.list_columns.items()}
        clean_column_names()
    def is_utcdatetime(self, col_name):
        """Check if the datetime is a UTC one."""
        from airflow.utils.sqlalchemy import UtcDateTime
        if col_name in self.list_columns:
            obj = self.list_columns[col_name].type
            return (
                isinstance(obj, UtcDateTime)
                or isinstance(obj, sqla.types.TypeDecorator)
                and isinstance(obj.impl, UtcDateTime)
            )
        return False
    filter_converter_class = UtcAwareFilterConverter
FieldConverter.conversion_table = (
    ('is_utcdatetime', DateTimeWithTimezoneField, AirflowDateTimePickerWidget),
) + FieldConverter.conversion_table