Split long sentence in two for style

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: gvanrossum

peps/pep-0718.rst

@@ -23,7 +23,7 @@ subscripted.
 Motivation
 ----------
 
-Currently, classes allow passing type annotations for generic containers, this
+Currently, classes allow passing type annotations for generic containers. This
 is especially useful in common constructors such as ``list``\, ``tuple`` and ``dict``
 etc.