docs: update Mermaid name and URLs, fix grammar

[gy-mate]

Mermaid.js is now just referenced as Mermaid on their website.

[netlify]

✅ Deploy Preview for slidev ready!

Name	Link
🔨 Latest commit	50c39e1
🔍 Latest deploy log	https://app.netlify.com/sites/slidev/deploys/677277ad9a0eb60008ceba32
😎 Deploy Preview	https://deploy-preview-1994--slidev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[pkg-pr-new]

Open in Stackblitz

create-slidev

npm i https://pkg.pr.new/slidevjs/slidev/create-slidev@1994

create-slidev-theme

npm i https://pkg.pr.new/slidevjs/slidev/create-slidev-theme@1994

@slidev/parser

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/parser@1994

@slidev/cli

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/cli@1994

@slidev/client

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/client@1994

@slidev/types

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/types@1994

commit: 50c39e1