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/gpui/src/platform/test/platform.rs

@@ -126,7 +126,7 @@ impl Platform for TestPlatform {
         #[cfg(target_os = "macos")]
         return Arc::new(crate::platform::mac::MacTextSystem::new());
 
-        // todo!("windows")
+        // todo("windows")
         #[cfg(target_os = "windows")]
         unimplemented!()
     }