docs: uploading-a-package walkthrough teaches a push pattern that raises

[drernie]

Spotted by @sir-sigurd while reviewing #5180.

docs/walkthrough/uploading-a-package.md teaches this for updating an existing package:

p = quilt3.Package()
p.push(
    "aneesh/test_data",
    "s3://quilt-example",
    message="Updated version my package"
)

A fresh Package() has no _origin, so for a package that already has revisions this raises QuiltConflictException — and has since 4.1.0 (Feb 2022), when the parent-hash check landed. The docs and the code have contradicted each other ever since.

The same shape appears three more times in that file (lines ~37, ~51, ~58).

The walkthrough should show one of the two routes that actually work:

• carry on from the package push() returned, or
• p = quilt3.Package.browse("aneesh/test_data", "s3://quilt-example") first.

Note this is only about the docs. #5180 fixes the related client bug (a second push from the same object) and expands the conflict message to name both safe routes, but it does not touch the walkthrough.