Update README.md

Author: chyyuu

README.md

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 # rCore-Tutorial-v3
 
-Welcome to JOIN [**Open-Source-OS-Training-Camp-2022 !**](https://learningos.github.io/rust-based-os-comp2022/)
+**Welcome to JOIN** [**Open-Source-OS-Training-Camp-2022 !**](https://learningos.github.io/rust-based-os-comp2022/)
 
 rCore-Tutorial version 3.6. See the [Documentation in Chinese](https://rcore-os.github.io/rCore-Tutorial-Book-v3/).