docs: Fix a typo in README and Contributing (#4264)

Author: danpls

Contributing.md

@@ -26,7 +26,7 @@ You can alternatively use `cargo` directly, but to do so you'll have to manually
 For example:
 ```sh
 export CFG_RELEASE=1.45.0-nightly
-export CFG_RELEASE=nightly
+export CFG_RELEASE_CHANNEL=nightly
 cargo +nightly build ...
 # And/or inline if you prefer, or need to override value(s)
 CFG_RELEASE_CHANNEL=nightly CFG_RELEASE=1.45.0-nightly cargo +nightly build ... 

README.md

@@ -94,7 +94,7 @@ Alternatively, you can run `cargo install` directly as long as you set the requi
 
 ```sh
 export CFG_RELEASE=1.45.0-nightly
-export CFG_RELEASE=nightly
+export CFG_RELEASE_CHANNEL=nightly
 cargo install --path . --force --locked --features rustfmt,cargo-fmt
 ```
 (Windows users can use `set` to specify the environment variable values)
@@ -164,7 +164,7 @@ You can alternatively use `cargo` directly, but you'll have to set the `CFG_RELE
 For example:
 ```sh
 export CFG_RELEASE=1.45.0-nightly
-export CFG_RELEASE=nightly
+export CFG_RELEASE_CHANNEL=nightly
 ```
 (Windows users can use `set` to specify the environment variable values)