chore(docs): replace "action" to "loader" in the login function comment

[liady]

WHY are these changes introduced?

Currently the comment for our CustomerAccount#login function specify it should be called and returned from an action, though it should be done from a loader - and this is what we also write in our docs.

WHAT is this pull request doing?

Fixes the wording in the comment.

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run npm run changeset add to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[shopify]

Oxygen deployed a preview of your chore/login-comment-fix branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 16, 2025 5:10 PM
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 16, 2025 5:10 PM
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 16, 2025 5:10 PM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 16, 2025 5:10 PM
classic-remix	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 16, 2025 5:10 PM

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