Update script help message

datasets/MEX/HRSC/download_sample.sh

@@ -35,6 +35,10 @@ function usage() {
   echo ""
   echo "    $ DRY_RUN=y ORBIT=0103 ./download_sample.sh data"
   echo ""
+  echo "  Download from NASA instead of ESA (default):"
+  echo ""
+  echo "    $ SRV=NASA ORBIT=0103 ./download_sample.sh data"
+  echo ""
 }
 
 SRV=${SRV:-ESA}