`source-repository head` points at the `main` branch

## Summary

Point the `source-repository head` stanza at `main`. It still names `develop`,
a branch that no longer exists, and the stale value is already published.

## Motivation

`collection-json.cabal:33-36`:

```cabal
source-repository head
type: git
location: https://github.com/alunduil/collection-json.hs
branch: develop
```

#82 renamed `develop` to `main` and retired `master` but missed the cabal file.
`gh api repos/:owner/:repo/branches` now lists `main` and no `develop`.

This is not just cosmetic drift in the working tree — the stanza ships in the
sdist, renders on the Hackage page, and drives `cabal get -s`, which is broken
against the currently published version:

```console
$ cabal get -s collection-json-1.3.1.3
Cloning into 'collection-json'...
fatal: Remote branch develop not found in upstream origin
Failed to fetch the source repository for package collection-json-1.3.1.3,
repository location https://github.com/alunduil/collection-json.hs
(git failed with ExitFailure 128).
```

The published 1.3.1.3 cabal file carries the same `branch: develop`, so this
regressed the moment the branch was renamed rather than at any upload. Anyone
reaching for the sources through cabal today hits it.

## Scope

- Change `branch: develop` to `branch: main` in `collection-json.cabal`.

## Acceptance criteria

- [ ] `source-repository head` names a branch that exists on the remote.
- [ ] `cabal check` stays clean.
- [ ] `cabal get -s` resolves against the sdist built from this tree.

## Additional context

- Missed by #82.
- Wants to land before #84 tags, so 1.3.2.0 is the upload that fixes it rather
than the second one carrying it.