fix eslint plugin install instructions in docs

[stefanprobst]

this pr fixes the install instructions in the docs for the Next.js ESLint plugin.

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sidenote: there are a few other places where the plugin is called eslint-plugin-next, but links to the correct @next/eslint-plugin-next, e.g.:

next.js/docs/basic-features/eslint.md

Line 73 in c6a08f5

- [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next)

not sure if intentional, or is the goal to eventually publish the plugin as eslint-plugin-next (which is owned by timer but currently empty)?

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Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes

[housseindjirdeh]

Ah thanks for this! The initial plan was to publish to eslint-plugin-next, but since that hasn't happened the docs shouldn't be incorrect. If we end up publishing to eslint-plugin-next, we can update accordingly :).