Update star formation history hdf5 properties docs

A change in unit was introduced, but the section of the documentation on
the star formation histories datasets wasn't updated with the changes.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

doc/hdf5_properties/star_formation_histories.rst

@@ -45,7 +45,7 @@
 ``run_info``
 ^^^^^^^^^^^^
 * ``batches``: number of batches analysed
-* ``effective_volume``: effective volume of this run [cMpc/h]
+* ``effective_volume``: effective volume of this run [(cMpc/h)^3]
 * ``lbox``: Box side size of the full simulated volume [Mpc/h]
 * ``ode_solver_precision``: accuracy applied when solving the ODE system of the physical model.
 * ``particle_mass``: dark matter particle mass of this simulation [Msun/h]