docs: fix warning markdown syntax (#428)

## Summary

Documentation:
- Correct warning admonition formatting in
[README.md](https://github.com/google-github-actions/run-gemini-cli/blob/main/README.md)
so it renders properly in Markdown viewers.

## Description

|
[Before](https://github.com/google-github-actions/run-gemini-cli/tree/main?tab=readme-ov-file#repository-variables)
|
[After](https://github.com/Marukome0743/run-gemini-cli/tree/docs/readme?tab=readme-ov-file#repository-variables)
|
|--------|--------|
| <img width="1113" height="274" alt="before"
src="https://github.com/user-attachments/assets/35b30cc5-ea19-4c3b-9d9b-dbd0611bc81c"
/> | <img width="1115" height="342" alt="after"
src="https://github.com/user-attachments/assets/2f8aaff7-8a63-423a-989a-17b524aa8757"
/> |

Author: Marukome0743

README.md

@@ -227,7 +227,8 @@ We recommend setting the following values as repository variables so they can be
 | `GOOGLE_GENAI_USE_GCA`      | Set to `true` to use Gemini Code Assist                                                                                        | Variable | No       | Using Gemini Code Assist       |
 | `APP_ID`                    | GitHub App ID for custom authentication.                                                                                       | Variable | No       | Using a custom GitHub App      |
 
-[!WARNING] Do not use the `DEBUG` environment variable as it causes the Gemini CLI to hang waiting for node debugger to attach.
+> [!WARNING]
+> Do not use the `DEBUG` environment variable as it causes the Gemini CLI to hang waiting for node debugger to attach.
 
 To add a repository variable: