fix example code in README

[ethe]

In current, the example code in README can not be compiled:

❯ hvm run hello.hvm
thread 'main' panicked at /Users/gwo/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hvm-2.0.15/src/main.rs:70:62:
PARSE_ERROR
- expected: name
- detected:
   7 |   &! @sum ~ (a (b $(:[+] $(e f))))

This PR will fix it.