Remove the stale link from powershell-beginners-guide.md

[daxian-dbw]

PR Summary

Remove the stale link from powershell-beginners-guide.md.
The stale link causes a failure in static analysis tests:

your last commit had 1 failures in `PowerShell-CI-static-analysis`
Verify Markdown Links.Verify links in /home/vsts/work/1/s/docs/learning-powershell/powershell-beginners-guide.md.https://examples.oreilly.com/9780596528492/ should work

retry of URL failed with error: Connection timed out
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

PR Context

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
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• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).

[ghost]

🎉v7.0.0-preview.6 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes