auto merge of #5788 : danluu/rust/rusti_clear, r=bstrie

The old help text reads "clear the screen", but :clear seems intended to do something completely different. I'm not sure what the intent is. If it's really supposed to clear the screen, not only does it not clear the screen, it does 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.

src/librusti/rusti.rc

@@ -273,7 +273,7 @@ fn run_cmd(repl: &mut Repl, _in: @io::Reader, _out: @io::Writer,
                 ~":{\\n ..lines.. \\n:}\\n - execute multiline command\n" +
                 ~":load <crate> ... - \
                   loads given crates as dynamic libraries\n" +
-                ~":clear - clear the screen\n" +
+                ~":clear - clear the bindings\n" +
                 ~":exit - exit from the repl\n" +
                 ~":help - show this message");
         }