Rename chat_with_functions to use snake_case (#11020)

This PR renames the `chat-with-functions.rs` example to use snake_case
for the filename, as is convention.

Release Notes:

- N/A

crates/assistant2/examples/chat_with_functions.rs

@@ -1,4 +1,5 @@
-/// This example creates a basic Chat UI with a function for rolling a die.
+//! This example creates a basic Chat UI with a function for rolling a die.
+
 use anyhow::{Context as _, Result};
 use assets::Assets;
 use assistant2::AssistantPanel;