Upgrade Rust version to 2021 for Chinese version

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chinese

rust-by-practice/zh-CN/book.toml

Line 22 in e0e88e9

edition = "2018"

English

rust-by-practice/en/book.toml

Line 22 in e0e88e9

edition = "2021"

Rust play ground version is already 2021 on English pages. Chinese version should have the same version.

And, some solutions don't work under 2018, e.g. this.