Fix attractors example parameter value

[jbednar]

The main fix is in https://github.com/pyviz-topics/examples/commit/970d0f8b9eeeb901ef761ccb94810891adb3d535 , where somehow a "2" in my original project https://anaconda.org/jbednar/project/attractors became a "-11", which is not a valid value for a Hopalong attractor.

Also reformatted the .yml so that it's easier to skim and edit, and fixed a warning from IPython for display.