yoda: use pencil-symbolic for the Rename toolbar button

document-edit-symbolic isn't in the Cosmic theme (checked Cosmic, Pop,
Adwaita, WhiteSur-dark ship lists) so it rendered empty. The Cosmic
theme ships pencil-symbolic which matches the edit/rename semantics
and is guaranteed to resolve through the current icon theme chain.

src/app.rs

@@ -6542,7 +6542,7 @@ impl Application for App {
             let toolbar = widget::row::with_children(vec![
                 tb_btn("folder-new-symbolic", fl!("new-folder"),
                        Action::NewFolder.message(None), true).into(),
-                tb_btn("document-edit-symbolic", fl!("rename"),
+                tb_btn("pencil-symbolic", fl!("rename"),
                        Action::Rename.message(None), true).into(),
                 tb_btn("edit-delete-symbolic", fl!("delete"),
                        Action::Delete.message(None), true).into(),