Fix incorrect return type doc
Fix incorrect return type doc

You can see it was incorrect by looking at RedirectPlugin::beforeSendPerformed
and at the implementations of the Swift_Mime_MimeEntity interface

fvosberg/swiftmailer,denpamusic/swiftmailer,c960657/swiftmailer,handautsav/swiftmailer,Dubz/swiftmailer,swiftmailer/swiftmailer,deguif/swiftmailer,gburyakovsky/swiftmailer,Dubz/swiftmailer,gencer/swiftmailer,papagel/swiftmailer,tweakers-dev/swiftmailer,michabbb-backup/swiftmailer,Rotzbua/swiftmailer,tweakers-dev/swiftmailer,jamesggordon/swiftmailer,kuslahne/swiftmailer,denpamusic/swiftmailer,jamesggordon/swiftmailer,zghosts/swiftmailer,michabbb-backup/swiftmailer,fvosberg/swiftmailer,Chhunlong/swiftmailer,voku/swiftmailer,gencer/swiftmailer,voku/swiftmailer,gburyakovsky/swiftmailer,papagel/swiftmailer,zghosts/swiftmailer,c960657/swiftmailer,deguif/swiftmailer,kuslahne/swiftmailer,robertklein/swiftmailer,Rotzbua/swiftmailer,handautsav/swiftmailer,swiftmailer/swiftmailer,Chhunlong/swiftmailer,robertklein/swiftmailer