Update cleanup instructions

.gitignore

@@ -1,7 +1,5 @@
 .idea/
 Cargo.lock
 target/
-cleanup
-cleanup.exe
 ab-results*.json
 results.yaml

src/bin/cleanup.rs

@@ -1,5 +1,5 @@
 // Cleans up `README.md`
-// Usage: rustc cleanup.rs && ./cleanup
+// Usage: cargo run --bin cleanup
 
 use std::fs;
 use std::fs::File;