fix RTD docs build (#8519)

Author: andersy005

doc/whats-new.rst

@@ -84,7 +84,7 @@ Documentation
 - Improved error message when attempting to get a variable which doesn't exist from a Dataset.
   (:pull:`8474`)
   By `Maximilian Roos <https://github.com/max-sixty>`_.
-- Fix default value of ``combine_attrs `` in :py:func:`xarray.combine_by_coords` (:pull:`8471`)
+- Fix default value of ``combine_attrs`` in :py:func:`xarray.combine_by_coords` (:pull:`8471`)
   By `Gregorio L. Trevisan <https://github.com/gtrevisan>`_.
 
 Internal Changes