Converting iso to img completes without creating the img file
Whenever a command line tool outputs it's Usage help text it means it couldn't parse your parameters. In this case you got the order of the arguments wrong, which can easily be gleaned from the man page (man hdiutil): convert image -format format -o outfile

In your case that'd be:

hdiutil convert ~/path/to/ubuntu.iso -format UDRW -o ~/path/to/target.img