docs: update base branch instructions to use pencil icon (#43423)

[teee32]

Fixes #43423\n\nUpdated the steps to reference the pencil icon next to the PR title instead of the outdated Edit button (screenshot removed to avoid keeping stale UI imagery).

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[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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[Copilot]

Pull request overview

Updates a pull request documentation page to reflect the current GitHub UI for changing a pull request’s base branch by referencing the pencil icon next to the title (and removing an outdated “Edit” button screenshot).

Changes:

• Replaced the “click Edit” instruction with guidance to use the pencil icon next to the pull request title.
• Removed the outdated screenshot that showed an Edit button.

[Sharra-writes]

@teee32 I'll get this merged after our deploy freeze is over.