PEP 661: Standardize module name (#2795)

standardize mod name

The PEP says that the module name is "sentinels" (plural) but the example imports Sentinel from the "sentinel" (singular) module.  Not absolutely sure which one is intended but recent standards (secrets, dataclasses) have used the plural.

CLA for Python signed, in case that's needed.

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

peps/pep-0661.rst

@@ -145,7 +145,7 @@ Its initializer will accept a single required argument, the name of the
 sentinel object, and two optional arguments: the repr of the object, and the
 name of its module::
 
-    >>> from sentinel import Sentinel
+    >>> from sentinels import Sentinel
     >>> NotGiven = Sentinel('NotGiven')
     >>> NotGiven
     <NotGiven>