Add react-router-dom install step to React tutorial

[fibibot]

The "Add a router" section in examples/tutorials/react.md jumped
straight into import { BrowserRouter, Route, Routes } from "react-router-dom"
without ever telling the reader to install the package. A user
following the tutorial verbatim hits an unresolved-import error as
soon as they save App.tsx — the package isn't in any deno add
command on the page, even though it's in the upstream
tutorial-with-react repo's deno.json.

This adds the missing deno add npm:react-router-dom step and a
short explanation of what BrowserRouter, Routes, and Route do,
plus how the :selectedDinosaur path parameter flows through to the
useParams() call in Dinosaur.tsx — which previously appeared
without motivation.

Verified by running deno add npm:react-router-dom against
denoland/deno:latest (deno 2.7.14) in a clean directory; it
resolves to react-router-dom@7.15.0 and writes a clean deno.json
imports entry. No code blocks were changed, so the rest of the
tutorial is unaffected.

Closes bartlomieju/orchid-inbox#46

[fibibot]

@bartlomieju ready for review when you have a moment.

[bartlomieju]

Looks good, remove em dashes

[lunadogbot]

CI is green and the added deno add npm:react-router-dom step matches the later bare react-router-dom imports. No blockers found.

[lunadogbot]

@bartlomieju this is ready to merge