Show how to remove positive activation map

[dgdillon]

It may be helpful to finish the example here https://pysurfer.github.io/examples/plot_fmri_activation.html by showing how to remove the "positive only" activation map. Following the example I tried "brain.overlays['sig'].remove()", which made sense b/c I'm plotting the sig.mgh file, but this crashed. Easy enough to figure out it's 'sig2', but wouldn't hurt to add it explicitly for new users.