Hv's example run results in error

[ShawnLi7777]

https://ubermag.github.io/api/_autosummary/discretisedfield.plotting.Hv.html
The example on the website has a running error：

import discretisedfield as df
p1 = (0, 0, 0)
p2 = (100, 100, 100)
n = (10, 10, 10)
mesh = df.Mesh(p1=p1, p2=p2, n=n)
field = df.Field(mesh, dim=1, value=2)
field.hv.contour(kdims=['y', 'z'])

The result after running is：

All other examples work fine.

[samjrholt]

We will change the example but expect the failure from trying to generate contours on a uniform field.