[Expressions] コードの記載方法、「Grammar of a ...」の表記方法の修正

[stzn]

https://github.com/apple/swift-book/blob/main/TSPL.docc/ReferenceManual/Expressions.md

原文に合わせた変更をしたい

• コードの記載を画像からMarkdown方式に変更

• Grammar of a ...の記述を原文の方法に合わせる

> GRAMMAR OF AN EXPRESSION  
> expression → [try-operator](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_try-operator)<sub>opt</sub> [prefix-expression](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_prefix-expression) [binary-expressions](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_binary-expressions)<sub>opt</sub>  
> expression-list → [expression](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_expression) \| [expression](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_expression) `,` [expression-list](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#grammar_expression-list)

を

> Grammar of an expression:
>
> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_
>
> *expression-list* → *expression* | *expression* **`,`** *expression-list*

にする