docs: Remove misplaced npm install

[tkindy]

I think this line was originally meant to be in the "Install React Router v6" section, but there's already an npm install command shown there.

[remix-cla-bot]

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[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[tkindy]

Or maybe this line I deleted should replace the npm install react-router@next that's in the "Install React Router v6" section? That one doesn't include react-router-dom which seems like a mistake.

[timdorr]

I got you on that. Thanks!