docs: fix how to update next minor upgrade branch

[kenjis]

Description

• the step to update local 4.x branch was missing

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• [] Conforms to style guide

[MGatner]

This is my fault for not looking closely. But I actually recommend working off the repo branches directly for release, as it makes the process faster and reduces mistakes:

Work off direct clones of the repos so the release branches persist for a time

So the upstream remote is irrelevant.

[MGatner]

It won't let me suggest a change for untouched lines, but please rework for direct branch access.

[kenjis]

Fixed the git commands.