MDC_#512 - Outdated comment mentions non-existing MDCButtonBar+Builder.h
MDC_#512 - Outdated comment mentions non-existing MDCButtonBar+Builder.h

Summary:
Replaced deprecated category mention of MDCButtonBar+Builder.h with updated class MDCAppBarButtonBarBuilder.h

Files Changed:
MDCButtonBar.h

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Closes https://github.com/google/material-components-ios/pull/597
GitHub author: ShepJGoogle

!![[https://github.com/google/material-components-ios/pull/597 | This is a child of GitHub pull request #597]]!!.

Reviewers: O1 Material components iOS, #mdc_ios_owners, github-bot

Reviewed By: O1 Material components iOS, #mdc_ios_owners

Tags: #material_components_ios

Differential Revision: http://codereview.cc/D1028

Eccelor/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,brianjmoore/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,samnm/material-components-ios