Fix muscle innervation test in the README doc tests

[slarson]

This test was taken out of the readme to get a build to work, but needs to be put back in

See what neurons innervate a muscle::

>>> mdr21 = P.Muscle('MDR21')
>>> innervates_mdr21 = mdr21.innervatedBy()
>>> len(innervates_mdr21)
4

[mwatts15]

To do this as written, requires to

1. add muscle.neurons (aliased to innervatedBy) relationships in addition to the existing Connections with muscle termination, or
2. write a property that does the query on Connections. This is essentially Neighbor in PyOpenWorm.neuron. Could even move neighbor up into cell.