Update README.md

Traceback (most recent call last):

  File "11-sherpa_table_a.py", line 35, in <module>
    prueba()
  File "11-sherpa_table_a.py", line 32, in prueba
    print(etree.tostring(link))
  File "lxml.etree.pyx", line 2871, in lxml.etree.tostring (src/lxml/lxml.etree.c:55650)
  File "serializer.pxi", line 131, in lxml.etree._tostring (src/lxml/lxml.etree.c:90378)
  File "serializer.pxi", line 186, in lxml.etree._raiseSerialisationError (src/lxml/lxml.etree.c:90939)
lxml.etree.SerialisationError: IO_ENCODER