Fix incorrect reference to "knumber" in Plotter editor
Fix incorrect reference to "knumber" in Plotter editor

Summary:
The Histogram editor doesn't seem to work due to a reference to "knumber". This seems to have been
broken for a while (maybe since D26613) -- wonder if there are any relevant support tickets out
there?

Test Plan:
- Open up a Histogram in the editor ([link](http://localhost:9000/#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20plotter%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22plotter%201%22%3A%7B%22type%22%3A%22plotter%22%2C%22alignment%22%3A%22default%22%2C%22static%22%3Afalse%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22correct%22%3A%5B1%5D%2C%22starting%22%3A%5B1%5D%2C%22type%22%3A%22bar%22%2C%22labels%22%3A%5B%22%22%2C%22%22%5D%2C%22categories%22%3A%5B%22%22%5D%2C%22scaleY%22%3A1%2C%22maxY%22%3A10%2C%22snapsPerLine%22%3A2%2C%22labelInterval%22%3A1%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22calculator%22%3Afalse%2C%22chi2Table%22%3Afalse%2C%22periodicTable%22%3Afalse%2C%22tTable%22%3Afalse%2C%22zTable%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)).
    - Verify that it renders.
    - Verify that there is no console error.

Auditors: emily

learningequality/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus