Removed mention of nonexistant `CSliderCtrl::VerifyPos`

[Avasam]

As far as I can tell, thanks to pywin32, CSliderCtrl::VerifyPos was removed in MFC 7.1

[prmerger-automator[bot]]

@Avasam : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod[bot]]

Learn Build status updates of commit e50b998:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/mfc/slider-control-member-functions.md	:white_check_mark:Succeeded

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[ttorble]

@TylerMSFT

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team