fix: Next.js missed a file when renaming swrDelta to expireTime (#71190)

# 🧰Fixed a file name

Hello. In #71159 Next.js changed `swrDelta` to `expireTime`. 

However, the `swrDelta.mdx` file name wasn't updated. So I renamed it to
`expireTime.mdx`.

Additionally, I fixed a typo. `stale-while-revalidate` is in an inline
code block in the paragraph below, but not in the one above. (Please see
this
[link](https://nextjs.org/docs/canary/app/api-reference/next-config-js/swrDelta).)


![image](https://github.com/user-attachments/assets/fdbd4db1-43f3-4c58-a842-55c51f282c5d)

Author: lumirlumir

docs/02-app/02-api-reference/05-next-config-js/swrDelta.mdx renamed to docs/02-app/02-api-reference/05-next-config-js/expireTime.mdx

@@ -5,7 +5,7 @@ description: Set a custom stale-while-revalidate period for ISR enabled pages.
 
 {/* The content of this doc is shared between the app and pages router. You can use the `<PagesOnly>Content</PagesOnly>` component to add content that is specific to the Pages Router. Any shared content should not be wrapped in a component. */}
 
-You can specify a custom stale-while-revalidate period for CDNs to consume in the `Cache-Control` header for ISR enabled pages.
+You can specify a custom `stale-while-revalidate` period for CDNs to consume in the `Cache-Control` header for ISR enabled pages.
 
 Open `next.config.js` and add the `expireTime` config: