debugger: Change placeholder text for Custom/Run text input (#31264)

Before: 

![image](https://github.com/user-attachments/assets/6cdef5bb-c901-4954-a2ec-39c59f8314db)

After:

![image](https://github.com/user-attachments/assets/c4f60a23-249c-47ab-8a9e-a39e2277dd00)


Release Notes:

- N/A

crates/debugger_ui/src/new_session_modal.rs

@@ -750,7 +750,10 @@ impl CustomMode {
 
         let program = cx.new(|cx| Editor::single_line(window, cx));
         program.update(cx, |this, cx| {
-            this.set_placeholder_text("Run", cx);
+            this.set_placeholder_text(
+                "ALPHA=\"Windows\" BETA=\"Wen\" your_program --arg1 --arg2=arg3",
+                cx,
+            );
 
             if let Some(past_program) = past_program {
                 this.set_text(past_program, window, cx);