docs: fix code block extension for `cacheLife` (#72171)
This PR modifies the code block extension for
[cacheLife](https://nextjs.org/docs/canary/app/api-reference/functions/cacheLife#usage).
As shown in the screenshot below, both code blocks were TSX
(TypeScript), so I followed the guidelines
[here](https://nextjs.org/docs/canary/community/contribution-guide#ts-and-js-switcher)
and changed one of them to JSX (JavaScript).
Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
