Update libstd example to include -Zfmt-debug

Author: johnthagen

README.md

@@ -198,7 +198,8 @@ host: x86_64-apple-darwin
 # Use that target triple when building with build-std.
 # Add the =std,panic_abort to the option to make panic = "abort" Cargo.toml option work.
 # See: https://github.com/rust-lang/wg-cargo-std-aware/issues/56
-$ RUSTFLAGS="-Zlocation-detail=none" cargo +nightly build -Z build-std=std,panic_abort \
+$ RUSTFLAGS="-Zlocation-detail=none -Zfmt-debug=none" cargo +nightly build \
+  -Z build-std=std,panic_abort \
   -Z build-std-features="optimize_for_size" \
   --target x86_64-apple-darwin --release
 ```