fix(flopy3_MT3DMS_examples.ipynb): wrong indices were indexed (#815)
fix(flopy3_MT3DMS_examples.ipynb): wrong indices were indexed (#815)

4th plot of p10 was showing concentration @ 750 days for layer 4, not concentration @ 1,000 days for layer 3 as labels indicated
aleaf/flopy,jentjr/flopy,aleaf/flopy,jentjr/flopy