Minor improvement of docstring for Dataset (#2904)

This might help to avoid confusion. data_vars is always a mapping, not a
mapping, a variable or a tuple.

Author: lumbric

xarray/core/dataset.py

@@ -343,7 +343,7 @@ def __init__(self, data_vars=None, coords=None, attrs=None,
         ----------
         data_vars : dict-like, optional
             A mapping from variable names to :py:class:`~xarray.DataArray`
-            objects, :py:class:`~xarray.Variable` objects or tuples of the
+            objects, :py:class:`~xarray.Variable` objects or to tuples of the
             form ``(dims, data[, attrs])`` which can be used as arguments to
             create a new ``Variable``. Each dimension must have the same length
             in all variables in which it appears.