Fix the discord badge in readme (#614)

The discord badge was not hyperlinking to the server properly due to
quotes in the link

README.md

@@ -3,7 +3,7 @@
 # WebLLM
 [![NPM Package](https://img.shields.io/badge/NPM_Package-Published-cc3534)](https://www.npmjs.com/package/@mlc-ai/web-llm)
 [!["WebLLM Chat Deployed"](https://img.shields.io/badge/WebLLM_Chat-Deployed-%2332a852)](https://chat.webllm.ai/)
-[![Join Discord](https://img.shields.io/badge/Join-Discord-7289DA?logo=discord&logoColor=white)]("https://discord.gg/9Xpy2HGBuD")
+[![Join Discord](https://img.shields.io/badge/Join-Discord-7289DA?logo=discord&logoColor=white)](https://discord.gg/9Xpy2HGBuD)
 [![Related Repository: WebLLM Chat](https://img.shields.io/badge/Related_Repo-WebLLM_Chat-fafbfc?logo=github)](https://github.com/mlc-ai/web-llm-chat/)
 [![Related Repository: MLC LLM](https://img.shields.io/badge/Related_Repo-MLC_LLM-fafbfc?logo=github)](https://github.com/mlc-ai/mlc-llm/)