Guides/Chunked.md: [proposed](proposal) link is a leftover placeholder

`Guides/Chunked.md` line 48 reads:

> The `chunks(ofCount:)` method was previously [proposed](proposal) for inclusion

The link target is the literal string `proposal`, so the rendered link 404s (`https://github.com/apple/swift-algorithms/blob/main/Guides/proposal`). It appears to be a placeholder that never got filled in with the actual Swift Evolution proposal URL — presumably [SE-0270's predecessor pitch, i.e. the standard-library proposal for `chunks(ofCount:)`](https://github.com/apple/swift-evolution/blob/main/proposals/0270-rangeset-and-collection-operations.md) or the corresponding forums pitch, whichever the maintainers intended.

Filed as requested by @glessard in https://github.com/apple/swift-algorithms/pull/277#issuecomment (review of the Keyed.md link fix). Happy to open a PR once the intended target is confirmed.

Open Guides/Chunked.md at line 48 and inspect the placeholder link. Confirm with the maintainers whether the intended target is the linked SE-0270 proposal or the corresponding forums pitch, then update the link and verify the rendered destination no longer 404s.

Written by the indexing model from the issue text.