Update instructions (#372)

<!-- This is an auto-generated comment: release notes by openai -->
### Summary by OpenAI

```
**Chore:**
- Updated the version of `coderabbitai/openai-pr-reviewer` from `latest` to `v1` in the README.md file.

> 🎉 Here's to the code that never rests,  
> Upgraded now to be the best.  
> From `latest` we part, to `v1` we sail,  
> On this ship of progress, we shall never fail! 🚀
```
<!-- end of auto-generated comment: release notes by openai -->

README.md

@@ -67,7 +67,7 @@ jobs:
   review:
     runs-on: ubuntu-latest
     steps:
-      - uses: coderabbitai/openai-pr-reviewer@latest
+      - uses: coderabbitai/openai-pr-reviewer@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
@@ -229,7 +229,7 @@ jobs:
   review:
     runs-on: ubuntu-latest
     steps:
-      - uses: coderabbitai/openai-pr-reviewer@latest
+      - uses: coderabbitai/openai-pr-reviewer@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}