Use plot_trajectory in broadcasting example

[willGraham01]

We should use the new plot_trajectory method in the broadcasting methods example.

Since these are introduced in 2 separate PRs (#394 and #397), just making a placeholder issue to remind myself to either:

• Update Introduce decorator for broadcasting 1D methods #397 if Plotting wrappers: Head Trajectory #394 goes in first.
• Update the example with another short PR if Introduce decorator for broadcasting 1D methods #397 goes in first.