Doc: Fix grammar on Ellipsis docs (library/constants.rst)

Co-authored-by: Éric <merwok@netwok.org>

Author: adorilson

Doc/library/constants.rst

@@ -66,8 +66,8 @@ A small number of constants live in the built-in namespace.  They are:
 .. data:: Ellipsis
 
    The same as the ellipsis literal "``...``", an object frequently used as a
-   placeholder of another values. Assignments to  ``Ellipsis`` are possible.
-   However, assignments to  ``...`` are illegal and raise a :exc:`SyntaxError`.
+   placeholder for another value. Assignment to  ``Ellipsis`` is possible, but
+   assignment to  ``...`` is not possible and raises a :exc:`SyntaxError`.
    ``Ellipsis`` is the sole instance of the :data:`types.EllipsisType` type.