Changed message for Mummy to be more clear

When a Pokemon's ability changes because of Mummy, it only quickly shows
what the ability was formerly in a small box on the screen, instead of an
in-game message. We use parentheses for messages that don't actually
appear in-game, so Mummy's message is changed to use parentheses instead.
Merge pull request #253 from ascriptmaster/master

Changed message for Mummy to be more clear