Fix Docusaurus JSX syntax error in balance README.md (#151)

Summary:
Pull Request resolved: #151

The Docusaurus build was failing with a JSX syntax error due to an improperly closed HTML `<img>` tag in the Downloads badge on line 15 of the README.md file.

When Docusaurus processes markdown files, it converts them to JSX, which requires all HTML tags to be either properly closed or self-closing. The Downloads badge was using raw HTML `[<img src="...">](...)` without a proper closing tag, causing the build to fail with:

```
SyntaxError: Expected corresponding JSX closing tag for <img>
```

This change converts the Downloads badge to use proper markdown syntax `[![...](...)](...)`, making it consistent with the other badges in the file and resolving the build error.

Differential Revision: D87176281

fbshipit-source-id: 1daf6e9e707b876bf0a21fdb0e95e925a6b54bac

Author: talgalili

README.md

@@ -12,7 +12,7 @@
 [![Deploy Website](https://github.com/facebookresearch/balance/actions/workflows/deploy-website.yml/badge.svg)](https://github.com/facebookresearch/balance/actions/workflows/deploy-website.yml?query=branch%3Amain)
 [![Release](https://github.com/facebookresearch/balance/actions/workflows/release.yml/badge.svg)](https://github.com/facebookresearch/balance/actions/workflows/release.yml?query=branch%3Amain)
 [![DOI](https://img.shields.io/badge/DOI-10.48550/arXiv.2307.06024-blue.svg)](https://doi.org/10.48550/arXiv.2307.06024)
-[<img src="https://pepy.tech/badge/balance">](https://pepy.tech/project/balance)
+[![Downloads](https://pepy.tech/badge/balance)](https://pepy.tech/project/balance)
 
 </div>