Fix negative lookahead example in middleware.md (#42320)

If you don't prefix `static` with `_next`, none of the JS will be able
to load.

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## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm build && pnpm lint`
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docs/advanced-features/middleware.md

@@ -95,10 +95,10 @@ export const config = {
     /*
      * Match all request paths except for the ones starting with:
      * - api (API routes)
-     * - static (static files)
+     * - _next/static (static files)
      * - favicon.ico (favicon file)
      */
-    '/((?!api|static|favicon.ico).*)',
+    '/((?!api|_next/static|favicon.ico).*)',
   ],
 }
 ```