docs: Fix simple typo, existant -> existent

There is a small typo in bitfield/tests/tests.py.

Should read `existent` rather than `existant`.

Author: timgates42

bitfield/tests/tests.py

@@ -31,7 +31,7 @@ def test_defaults(self):
         self.assertEqual(int(bithandler.FLAG_1.number), 1)
         self.assertEqual(int(bithandler.FLAG_2.number), 2)
         self.assertEqual(int(bithandler.FLAG_3.number), 3)
-        # Negative test non-existant key.
+        # Negative test non-existent key.
         self.assertRaises(AttributeError, lambda: bithandler.FLAG_4)
         # Test bool().
         self.assertEqual(bool(bithandler.FLAG_0), False)