rollup merge of rust-lang#24398: steveklabnik/thanks_mdinger

The two-step command left off the nightly flag :frown:

Thanks @mdinger

Author: alexcrichton

src/doc/trpl/nightly-rust.md

@@ -18,7 +18,7 @@ use a two-step version of the installation and examine our installation script:
 
 ```bash
 $ curl -f -L https://static.rust-lang.org/rustup.sh -O
-$ sudo sh rustup.sh
+$ sudo sh rustup.sh --channel=nightly
 ```
 
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