Fix typo in documentation (fixes #572)
Fix typo in documentation (fixes #572)

Thanks @neuhaus!

qpdf/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf