Fix a docstring typo in _fill_mi_header (pandas-dev#15918)

[ci skip]

Author: crayxt

pandas/io/excel.py

@@ -571,7 +571,7 @@ def _fill_mi_header(row, control_row):
  ----------
  row : list
  List of items in a single row.
- constrol_row : list of boolean
+ control_row : list of boolean
  Helps to determine if particular column is in same parent index as the
  previous value. Used to stop propagation of empty cells between
  different indexes.