Update 34.ERC721 readme

[linghuccc]

What type of PR is this (这是什么类型的PR)

• Typo(勘误)
• BUG(程序错误)
• Improvement(提升)
• Feature(新特性)

Which issue(s) this PR fixes(Optional) (这个PR 修复了什么问题 (可选择))

• resolve(修复) : # fixed the typo

What this PR does / why we need it (这个PR 做了什么/ 我们为什么需要这个PR)

[XdpCs]

Can you add a link to explain the solmate? It will be more clearly.
The link is https://github.com/transmissions11/solmate/blob/main/src/tokens/ERC721.sol

[linghuccc]

Can you add a link to explain the solmate? It will be more clearly. The link is https://github.com/transmissions11/solmate/blob/main/src/tokens/ERC721.sol

Link added.