Hey, thanks - it's a valid concern, though an important point (which isn't really well explained in the document) is that this payload will work even if the tag isn't properly terminated.
One of the reasons for using such broken payloads is to demonstrate that browsers will happily parse broken markup and that approaches such as removing "<.*>" won't be effective as a technique to prevent XSS (because such a regexp won't match an unterminated tag like the example you pointed out).
Still, it could at least use a better explanation. The documentation fairy will take a look!
What payload are you using on that level? Keep in mind that ";" is often treated as a parameter separator in URLs, similarly to &. If you put it into the mock URL bar it will terminate the value of your parameter (see also http://en.wikipedia.org/wiki/Query_string#Web_forms)
One of the reasons for using such broken payloads is to demonstrate that browsers will happily parse broken markup and that approaches such as removing "<.*>" won't be effective as a technique to prevent XSS (because such a regexp won't match an unterminated tag like the example you pointed out).
Still, it could at least use a better explanation. The documentation fairy will take a look!