Cautionary accidental for All The Things You Are

Despite b flat being in the key signature, this is the only time in the entire
song that there is a b flat in the melody. B naturals appear throughout so
I think it's worth clarifying for those reading at sight..
Correcting notation of cautionary accidental
Merge pull request #66 from captbaritone/all-the-things-you-are-cautionary

Cautionary accidental for All The Things You Are