Tweak wording

Signed-off-by: Yuki Okushi <[email protected]>

Author: JohnTitor

README.md

@@ -24,7 +24,7 @@ Contributing to Glacier is fairly easy:
 3. Create a file in `ices/` with the same number as the issue reporting the ICE.
 4. Copy the code that causes the ICE into your new file.
 5. (optional) Verify it works by running `rustup update nightly`, then
-   `cargo run NUMBER` to run your ICE.
+   `cargo run $ISSUE_NUMBER` to run your ICE.
 6. Send a pull request!
 
 Note: Running this on Windows may give false positives and report some ICEs as fixed,