DOC: change colormap to gray

1. `jet` is generally discouraged as perceptually misleading: https://github.com/matplotlib/matplotlib/issues/875#issue-4616667
2. I think it's more clear when all 3 images use the same colormap, to show how an edge detector converts a solid block of brightness into bright lines on just the edges:

```
          ______
    _____/      \_____  -->  _____/\____/\_____
```