docs(next/router): capitalize React (#45950)

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm build && pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Author: valentincostam

docs/api-reference/next/router.md

@@ -140,7 +140,7 @@ export default function Page() {
 
 #### Resetting state after navigation
 
-When navigating to the same page in Next.js, the page's state **will not** be reset by default as react does not unmount unless the parent component has changed.
+When navigating to the same page in Next.js, the page's state **will not** be reset by default as React does not unmount unless the parent component has changed.
 
 ```jsx
 // pages/[slug].js