Fix CI instructions in README.md
Closes
@@ -162,7 +162,7 @@ language: rust
cache: cargo
before_script:
- export PATH="$PATH:$HOME/.cargo/bin"
-- which rustfmt || cargo install rustfmt
+- which rustfmt || cargo install rustfmt-nightly
script:
- cargo fmt -- --write-mode=diff
- cargo build
