docs: make badges center (#789)

README.md

 <div align="center">
 <img src="https://raw.githubusercontent.com/sgl-project/sglang/main/assets/logo.png" alt="logo" width="400"></img>
-</div>
 
 [![PyPI](https://img.shields.io/pypi/v/sglang)](https://pypi.org/project/sglang)
 ![PyPI - Downloads](https://img.shields.io/pypi/dm/sglang)
@@ -8,6 +7,8 @@
 [![issue resolution](https://img.shields.io/github/issues-closed-raw/sgl-project/sglang)](https://github.com/sgl-project/sglang/issues)
 [![open issues](https://img.shields.io/github/issues-raw/sgl-project/sglang)](https://github.com/sgl-project/sglang/issues)
 
+</div>
+
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 | [**Documentation**](https://sglang.readthedocs.io) | [**Blog**](https://lmsys.org/blog/2024-07-25-sglang-llama3/) | [**Paper**](https://arxiv.org/abs/2312.07104) | [**Slack**](https://join.slack.com/t/sgl-fru7574/shared_invite/zt-2ngly9muu-t37XiH87qvD~6rVBTkTEHw) |