docs: TNGLab path adjustment

[dnelson86]

Under the details of "supported_datasets/tng", you describe how to adjust the demo code from

ds = load("./snapdir_030")

to

sim = load("TNG50-4")
ds = sim.get_dataset(30)

which is good, I would leave this. But add comments that:

1. this works because scida automatically searches a list of known paths for "TNG50-4".
2. before this snippet, add a more explicit and different option sim = load("/home/tnguser/sims.TNG/TNG50-4/output/snapdir_030") which makes the connection to the first snippet very clear.