Clear TapTreeIter clippy warning

Clippy emits warning:

 public documentation for `script_leaves` links to private item `TapTreeIter`

I'm not exactly sure why this is but adding the generic type place
holder clears the warning.

Author: tcharding

src/util/psbt/map/output.rs

@@ -168,7 +168,7 @@ impl TapTree {
         self.0.clone()
     }
 
-    /// Returns [`TapTreeIter`] iterator for a taproot script tree, operating in DFS order over
+    /// Returns [`TapTreeIter<'_>`] iterator for a taproot script tree, operating in DFS order over
     /// tree [`ScriptLeaf`]s.
     pub fn script_leaves(&self) -> TapTreeIter {
         match (self.0.branch().len(), self.0.branch().last()) {