Remove ! from todo!() in comments (#8643)

This practice makes it difficult to locate todo!s in my code when I'm
working. Let's take out the bang if we want to keep doing this.

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -774,7 +774,7 @@ impl AssistantPanel {
             } else {
                 editor.highlight_background::<PendingInlineAssist>(
                     background_ranges,
-                    |theme| theme.editor_active_line_background, // todo!("use the appropriate color")
+                    |theme| theme.editor_active_line_background, // todo("use the appropriate color")
                     cx,
                 );
             }