Added issue 3351: Comments in MIMEDocument erroneously refer to MIMEObject

Pharo image: Pharo 

Pharo core version: 1.1-11411

Virtual machine used: n/a

Class browser used (if applicable): n/a

In the following method, comments refer to MIMEObject (which I assume is an earlier name for MIMEDocument):

   MIMEDocument class>>#contentType:content

I realize this is a small thing, but it did cause me to hunt around for MIMEObject for a while.