add in indication that next method is provided in guide level explanation

Signed-off-by: Nell Shamrell <[email protected]>

Author: nellshamrell

rfc-drafts/stream.md

@@ -73,6 +73,7 @@ pub trait Stream {
     fn next(&mut self) -> Next<'_, Self>
     where
         Self: Unpin;
+    { ... }
 }
 ```