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Fixes #43423\n\nUpdated the steps to reference the pencil icon next to the PR title instead of the outdated Edit button (screenshot removed to avoid keeping stale UI imagery).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GitHub UI for changing a pull request’s base branch by referencing the pencil icon next to the title (and removing an outdated “Edit” button screenshot).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• Removed the outdated screenshot that showed an Edit button.

[Copilot]

The UI element here is an icon with tooltip text (currently "Edit title"), so "click the pencil icon to edit the pull request" is a bit vague/inaccurate and may be confusing. Consider using the site’s standard octicon markup (e.g., {% octicon "pencil" ... %}) and wording that matches the UI label/location (e.g., "click the Edit title (pencil) icon next to the pull request title") to make the step clearer and consistent with other docs pages.

Suggested change

	1. Next to the pull request's title, click the pencil icon to edit the pull request.
	1. Next to the pull request title, click **Edit title** {% octicon "pencil" aria-label="Edit title" %}.