I think it will depend on your actual circumstances. If you failed to match into psych the first time and you're applying to the same programs again, you can't pretend you're suddenly deciding to apply. Instead you just explain why you want to be a psychiatrist and hope you're more competitive this time.
If you applied for FM and now are applying for psych for the first time, no one will think you are indecisive. They will think you are seeing the light. Most psychiatric educators wouldn't imagine someone wanting to do anything other than psych. Just provide a compelling story of why you want to be a psychiatrist.