Fix output paths for updates in Ch. 20

Author: chriskrycho

listings/ch20-advanced-features/listing-20-10/output.txt

@@ -1,4 +1,6 @@
 $ cargo +nightly miri run
+Preparing a sysroot for Miri (target: aarch64-apple-darwin)... done
+   Compiling unsafe-example v0.1.0 (file:///projects/unsafe-example)
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
      Running `/Users/chris/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/cargo-miri runner target/miri/aarch64-apple-darwin/debug/unsafe-example`
 warning: creating a shared reference to mutable static is discouraged