docs: fix invalid links (#1061)

## Summary
Updates the `tsdown` documentation links in `docs/guide/pack.md` to
point to the correct pages for:
- output formats
- watch mode

## Why
The previous links were invalid, which sent readers to 404 pages.

## Testing
- Open links
  - [Output Format](https://tsdown.dev/options/output-format)
  - [Watch Mode](https://tsdown.dev/options/watch-mode)
- Docs-only change; no code or runtime behavior changed

Author: bhbs

docs/guide/pack.md

@@ -25,8 +25,8 @@ See the [tsdown guide](https://tsdown.dev/guide/) and the [tsdown config file do
 Use it for:
 
 - [declaration files (`dts`)](https://tsdown.dev/options/dts)
-- [output formats](https://tsdown.dev/options/format)
-- [watch mode](https://tsdown.dev/options/watch)
+- [output formats](https://tsdown.dev/options/output-format)
+- [watch mode](https://tsdown.dev/options/watch-mode)
 - [standalone executables](https://tsdown.dev/options/exe#executable)
 
 ```ts