NEXT-66966 fix title inconsistency with `unstable_` functions in Docs (#66967)
add (Experimental) to title for `unstable_rethrow` to match all other
`unstable_` functions
### Fixing a bug
- Related issues linked using `fixes #66966`
<img width="1287" alt="image"
src="https://github.com/vercel/next.js/assets/147033096/5a790b63-30c9-4605-b0e8-73899afb8e59">
## For Maintainers
`unstable_rethrow` has a different title scheme compared to all other
`unstable_` functions this is just a quick fix for this since it was
bugging me.
Closes NEXT-66966
Fixes #66966
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Co-authored-by: Sam Ko <sam@vercel.com>