update BuilderArena comment

Author: dwrensha

capnp/src/private/arena.rs

@@ -155,10 +155,10 @@ where
 
 pub trait BuilderArena: ReaderArena {
     // These methods all take an immutable &self because otherwise a StructBuilder<'a>
-    // would need a `&'a mut BuilderArena` and `StructBuilder::borrow()` would
-    // have lifetime issues. (If `'a: 'b`, then a `&'a (BuilderArena + 'a)` can be
-    // converted to a `&'b (BuilderArena + 'b)`, but a `&'a mut (BuilderArena + 'a)`
-    // *cannot* be converted to a `&'b mut (BuilderArena + 'b)`. See some discussion here:
+    // would need a `&'a mut dyn BuilderArena` and `StructBuilder::reborrow()` would
+    // have lifetime issues. (If `'a: 'b`, then a `&'a dyn (BuilderArena + 'a)` can be
+    // converted to a `&'b dyn (BuilderArena + 'b)`, but a `&'a mut dyn (BuilderArena + 'a)`
+    // *cannot* be converted to a `&'b mut dyn (BuilderArena + 'b)`. See some discussion here:
     // https://botbot.me/mozilla/rust/2017-01-31/?msg=80228117&page=19 .)
     fn allocate(&self, segment_id: u32, amount: WordCount32) -> Option<u32>;
     fn allocate_anywhere(&self, amount: u32) -> (SegmentId, u32);