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

[alunduil]

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:

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:

$ 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 Rename develop → main and retire master #82.
• Wants to land before Cut 1.3.2.0 release #84 tags, so 1.3.2.0 is the upload that fixes it rather
  than the second one carrying it.