Fix wrong error message in interp() (#2598)

Author: lumbric

xarray/core/dataset.py

@@ -1946,7 +1946,7 @@ def interp(self, coords=None, method='linear', assume_sorted=False,
         """
         from . import missing
 
-        coords = either_dict_or_kwargs(coords, coords_kwargs, 'rename')
+        coords = either_dict_or_kwargs(coords, coords_kwargs, 'interp')
         indexers = OrderedDict(self._validate_indexers(coords))
 
         obj = self if assume_sorted else self.sortby([k for k in coords])