extract_headers() was throwing an exception when it was examining a l…

[pfrench42]

extract_headers() was throwing an exception when it was examining a line which read "hr". This fix ensures only hN lines which have digits are parsed

> /gpt_researcher/master/actions.py(331)extract_headers()
-> level = int(line[2])  # Extract header level
(Pdb) p line
'<hr />'
(Pdb) 

[pfrench42]

To be clear, there were \<hr> markers in the output which was causing line 331 to throw an exception and abort before research completion.