**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
self.check_generate_pages_html(current_page=0, total_pages=6)
    def test_generate_pager_current_middle(self):
        self.check_generate_pages_html(current_page=10, total_pages=20, check_middle=True)
    def test_generate_pager_current_end(self):
        self.check_generate_pages_html(current_page=38, total_pages=39)
    def test_params_no_values(self):
        """Should return an empty string if no params are passed"""
        assert '' == utils.get_params()
    def test_params_search(self):
        assert 'search=bash_' == utils.get_params(search='bash_')
    def test_params_none_and_zero(self):
        query_str = utils.get_params(a=0, b=None, c='true')
        
        pairs = list(sorted(query_str.split('&')))
        assert ['a=0', 'c=true'] == pairs
    def test_params_all(self):