Add a comment to ensure that a test won't be removed [ci skip]

The 'cow' => 'kine' inflection has gone with c300dca9 but it should
not be removed from the tested irregularities since it ensures that
inflections work with words that do not begin with the same letters.

activesupport/test/inflector_test_cases.rb

@@ -314,7 +314,7 @@ module InflectorTestCases
     'child'  => 'children',
     'sex'    => 'sexes',
     'move'   => 'moves',
-    'cow'    => 'kine',
+    'cow'    => 'kine', # Test inflections with different starting letters
     'zombie' => 'zombies',
     'genus'  => 'genera'
   }