Update rusti help text (for :clear) to match behavior

The old help text reads "clear the screen", but :clear seems intended to do something completely different.

```
rusti> fn foo() { println("called foo!") }
()
rusti> foo();
called foo!
()
rusti> :clear
rusti> foo();
<anon>:34:0: 34:3 error: unresolved name: `foo`.
<anon>:34 foo();
```

Per the contributor guidelines, here's the reason there's no testcase: it's a comment string.