gh-126055: Add omitted command (in docs [os.walk]) for code to fulfill shutil.rmtree algorithm (GH-126067)

* gh-126055:  Add omitted command (in docs [os.walk]) for code to fulfill `shutil.rmtree` algorithm.

Resolves #126055

* gh-126055:  Fix omitted code highlighting

Doc/library/os.rst

@@ -3675,6 +3675,7 @@ features:
               os.remove(os.path.join(root, name))
           for name in dirs:
               os.rmdir(os.path.join(root, name))
+      os.rmdir(top)
 
    .. audit-event:: os.walk top,topdown,onerror,followlinks os.walk