Fix the FileIO.closed docstring assertion in test_descr

Author: serhiy-storchaka

Lib/test/test_descr.py

@@ -3405,7 +3405,7 @@ def test_descrdoc(self):
         from _io import FileIO
         def check(descr, what):
             self.assertEqual(descr.__doc__, what)
-        check(FileIO.closed, "True if the file is closed") # getset descriptor
+        check(FileIO.closed, "True if the file is closed.") # getset descriptor
         check(complex.real, "the real part of a complex number") # member descriptor
 
     def test_doc_descriptor(self):