test_threaded_import: Fix unittest.main spelling (GH-93114)

(cherry picked from commit 19710145)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

Lib/test/test_importlib/test_threaded_import.py

@@ -270,4 +270,4 @@ def setUpModule():
 
 
 if __name__ == "__main__":
-    unittets.main()
+    unittest.main()