To export the wiki to a single file exactly as it is:
--render "$:/core/save/all"
That doesn't work so great, though, because it includes the plugins that sync with the node.js server, which means you'll get a bunch of errors when you try to view it anywhere else. Better is to use the --build command as Mark says, which will take care of extracting that stuff for you.
--build just executes a series of other commands found in the "build" section of the
tiddlywiki.info file. By default there is an "index" target in there that does what you want. If you're curious to see exactly what's going on, or you need to tweak something, take a look there.