fix: use pull request head sha for diffs

[hazre]

Fixes the pull request head SHA lookup so fork PRs diff against the actual head commit. There was a typo in context2.payload.pull_reuqest?.head.sha.

I noticed it while trying to add the action in a codebase.

[github-actions]

⚠️ Duplicate Dependencies (found: 3, threshold: 1)

📦 Package	📋 Versions
@actions/http-client	2 versions @e18e/action-dependency-diff@0.0.1 @actions/github@9.1.0 @actions/http-client@3.0.2 @e18e/action-dependency-diff@0.0.1 @actions/core@3.0.0 @actions/http-client@4.0.0
eslint-visitor-keys	2 versions @e18e/action-dependency-diff@0.0.1 eslint@10.2.0 @eslint-community/eslint-utils@4.9.1 eslint-visitor-keys@3.4.3 @e18e/action-dependency-diff@0.0.1 eslint@10.2.0 eslint-visitor-keys@5.0.1
ignore	2 versions @e18e/action-dependency-diff@0.0.1 eslint@10.2.0 ignore@5.3.2 @e18e/action-dependency-diff@0.0.1 typescript-eslint@8.58.1 @typescript-eslint/eslint-plugin@8.58.1 ignore@7.0.5

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