Update router.md to improve a11y (#23611)
Update router.md to improve a11y (#23611)

Code examples used `span`, change to use `button` for better accessibility 

## Documentation / Examples

- [x] Make sure the linting passes

azukaru/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js,flybayer/next.js,flybayer/next.js