docs: update npm install command

[chenxsan]

Changes

• Fix documentation

I believe astro should be installed under devDependencies instead of dependencies .

Testing

Not needed.

Docs

Not needed.

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[jasikpark]

I believe it should be under dependencies since it is required to build the project, rather than being a tool used for development of the website you might work on - https://docs.npmjs.com/cli/v7/configuring-npm/package-json#devdependencies

For example, Netlify does not install devDependencies. (I believe that to be true but cannot find it https://docs.netlify.com/configure-builds/manage-dependencies/)

[FredKSchott]

Thanks for the contribution! I actually personally prefer installing Astro as a dev dependency, since I think it would fall under the title of "tool used to build your website". BUT, I understand that not everyone agrees on this, and ultimately in most projects it's a non-issue that falls under personal preference.

Similar to how we show npm install and assume that yarn/pnpm users know how to map this to their own CLI, I think anyone who wants to install as a dev dependencies knows to add the --save-dev flag.