Merge pull request #163 from noi/rustup-override-set

Fix command to set toolchain

Author: tatsuya6502

src/appendix-07-nightly-rust.md

@@ -318,7 +318,7 @@ nightly toolchain as the one `rustup` should use when you’re in that directory
 
 ```text
 $ cd ~/projects/needs-nightly
-$ rustup override add nightly
+$ rustup override set nightly
 ```
 
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