DOC: formatters in DataFrame.to_html cannot be list
```
pandas\core\frame.pyc in to_html(self, buf, columns, col_space, colSpace, header, index, na_rep, formatters, float_format, sparsify, index_names, justify, force_unicode, bold_rows, classes)
   1537                                            header=header, index=index,
   1538                                            bold_rows=bold_rows)
-> 1539         formatter.to_html(classes=classes)
   1540
   1541         if buf is None:

pandas\core\format.pyc in to_html(self, classes)
    374         """
    375         html_renderer = HTMLFormatter(self, classes=classes)
--> 376         html_renderer.write_result(self.buf)
    377
    378     def _get_formatted_column_labels(self):

pandas\core\format.pyc in write_result(self, buf)
    549             indent += self.indent_delta
    550             indent = self._write_header(indent)
--> 551             indent = self._write_body(indent)
    552
    553         self.write('</table>', indent)

pandas\core\format.pyc in _write_body(self, indent)
    635         fmt_values = {}
    636         for i in range(len(self.columns)):
--> 637             fmt_values[i] = self.fmt._format_col(i)
    638
    639         # write values

pandas\core\format.pyc in _format_col(self, i)
    363     def _format_col(self, i):
    364         col = self.columns[i]
--> 365         formatter = self.formatters.get(col)
    366         return format_array(self.frame.icol(i).values, formatter,
    367                             float_format=self.float_format,

AttributeError: 'list' object has no attribute 'get'
```
