Update README.md

The repository name in the README.md file doesn't match the actual repositor name.
```
# 下载
- git clone --recursive git@github.com:decaf-lang/minidecaf-2023.git
+ git clone --recursive git@github.com:decaf-lang/minidecaf-start-code-2023.git
# 配置
- cd minidecaf-2023
+ cd minidecaf-start-code-2023
```