Update dialoghelper.rst

Consistent use of $this->getHelperSet()
minor #3842 Update dialoghelper.rst (bijsterdee)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3842).

Discussion
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Update dialoghelper.rst

Consistent use of $this->getHelperSet()

Commits
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2734511 Update dialoghelper.rst