book/development: Use +nightly for cargo fmt.

book/src/development.md

@@ -9,7 +9,7 @@ this by creating a git `pre-commit` hook like the following:
 
 ```shell
 #!/bin/sh
-if ! cargo fmt -- --check >/dev/null; then
+if ! cargo +nightly fmt -- --check >/dev/null; then
     echo "There are some code style issues. Run 'cargo fmt' to fix it."
     exit 1
 fi