PR: Fix error when showing context menu on Help plugin

[goanpeca]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

The browser widget was missing a call to the setup() method.

Issue(s) Resolved

Fixes #13422

Affirmation

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and understand I am releasing the same under Spyder's MIT (Expat) license.

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[ccordoba12]

/show binder

[github-actions]

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[ccordoba12]

Thanks @goanpeca!