update base branch edit UI reference

[XiaoPengMei]

Closes #43423

Summary

• update the base-branch editing instructions to reference the pencil icon instead of the removed Edit button
• remove the outdated screenshot that showed the old button

[github-actions]

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Source	Review	Production	What Changed
pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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