message: remove mention of the `im` crate
diff: @@ -74,6 +74,5 @@ and we cloned them quite frequently. This will work just fine in Salsa,
  but it may not be the most efficient choice. This is because each clone
  is going to produce a deep copy of the result. As a simple fix, you
  might convert your data structures to use `Arc` (e.g., `Arc<Vec<ParsedHeader>>`),
  -which makes cloning cheap. Or you might prefer to adopt persistent data
  -structures such as those found in [the `im` crate](https://crates.io/crates/im).
  +which makes cloning cheap.
  