Typo in Universal Functions section (#3663)

Author: yohai

doc/api.rst

@@ -410,7 +410,7 @@ Universal functions
    for the ``xarray.ufuncs`` module, which should not be used for new code
    unless compatibility with versions of NumPy prior to v1.13 is required.
 
-This functions are copied from NumPy, but extended to work on NumPy arrays,
+These functions are copied from NumPy, but extended to work on NumPy arrays,
 dask arrays and all xarray objects. You can find them in the ``xarray.ufuncs``
 module: