Fix click-to-add-points

Summary:
I needed to update the coords props after pushing the newly created
point onto the points array.

Test Plan:
Answer and grade the following without moving points after placing them,
and make sure it grades correctly:
http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22**Plot%20the%20real%20zeros%20of%20the%20given%20polynomial%20on%20the%20graph%20below.**%5Cn%5Cn%24%5C%5Cqquad%20p(x)%3D8x%5E3-20x%5E2-2x%2B5%24%5Cn%5Cn%5Cn%5B%5B%E2%98%83%20interactive-graph%201%5D%5D%20%22%2C%22widgets%22%3A%7B%22interactive-graph%201%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B1%2C1%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22range%22%3A%5B%5B-5%2C5%5D%2C%5B-5%2C5%5D%5D%2C%22gridStep%22%3A%5B0.5%2C0.5%5D%2C%22snapStep%22%3A%5B0.25%2C0.25%5D%2C%22graph%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%2C%22coords%22%3A%5B%5B-0.5%2C0%5D%2C%5B0.5%2C0%5D%2C%5B2.5%2C0%5D%5D%7D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Plot%20the%20points.%22%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22The%20%20zeros%20of%20%24p(x)%24%20are%20those%20values%20of%20%24x%24%20for%20which%20%24p(x)%3D0%24.%20%20So%20let's%20factor%20%24p(x)%24%20and%20solve%20the%20equation%20%24p(x)%3D0%24.%5Cn%5Cn%24%5C%5Cqquad%20%5C%5Cbegin%7Balign%7D8x%5E3-20x%5E2-2x%2B5%26%3D4x%5E2(2x-5)-1(2x-5)%5C%5C%5C%5C%26%3D(4x%5E2-1)(2x-5)%5C%5C%5C%5C%26%3D(2x-1)(2x%2B1)(2x-5)%5C%5C%5C%5C%26%3D0.%5C%5Cend%7Balign%7D%24%5Cn%5CnLet's%20set%20each%20factor%20equal%20to%20%240%24.%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22If%20%242x-1%3D0%24%2C%20then%20%24x%3D%5C%5Cdfrac%7B1%7D%7B2%7D%24.%5Cn%5CnIf%20%242x%2B1%3D0%24%2C%20then%20%24x%3D-%5C%5Cdfrac%7B1%7D%7B2%7D%24.%5Cn%5CnIf%20%242x-5%3D0%24%2C%20then%20%24x%3D%5C%5Cdfrac%7B5%7D%7B2%7D%24.%5Cn%5CnThe%20real%20zeros%20are%20%24x%3D%5C%5Cdfrac%7B1%7D%7B2%7D%2C-%5C%5Cdfrac%7B1%7D%7B2%7D%24%2C%20and%20%24%5C%5Cdfrac%7B5%7D%7B2%7D%24.%20%20%5Cn%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20real%20zeros%20are%20%24x%3D%5C%5Cdfrac%7B1%7D%7B2%7D%2C-%5C%5Cdfrac%7B1%7D%7B2%7D%24%2C%20and%20%24%5C%5Cdfrac%7B5%7D%7B2%7D%24%2C%20so%20we%20plot%20the%20points%20%24(%5C%5Cdfrac%7B1%7D%7B2%7D%2C0)%24%2C%20%24(-%5C%5Cdfrac%7B1%7D%7B2%7D%2C0)%24%2C%20%20and%20%24(%5C%5Cdfrac%7B5%7D%7B2%7D%2C0)%24.%5Cn%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F672c2d65c36dcd51da45f550563f8dc7b6dfd22f.png)%5Cn%5Cn%5Cn%5Cn%5Cn%5Cn%5Cn%5Cn%5Cn%22%2C%22widgets%22%3A%7B%7D%7D%5D%7D

Auditors: eater