DOC: fixed example for del_vertices/3.
DOC: fixed example for del_vertices/3.

edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,koryonik/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel