[MenuBundle] Fix typo "an class"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | N/A

http://public.wsu.edu/~brians/errors/a.html
https://en.wikipedia.org/wiki/English_articles#Distinction_between_a_and_an