Update README.md

datasets/README.md

 # Datasets
 
-## MEX
-
-### HRSC
+## MEX / HRSC
 
 FTP links for Mars Express missions can be found at:
 https://www.cosmos.esa.int/web/psa/ftp-browser#mex
 
 
-## MRO
-
-### CRISM
+## MRO / CRISM
 
 We start at the [PDS Geosciences Node] page for [MRO CRISM] data access.