ansis:SoilSite - update purpose def

[meganrwong]

ontology -
anais:purpose definition can be updated to as per schema -
from -
"description": "Purpose of or reason for activity.",
to-
"description": "Purpose of or reason for a site or activity. For example, why a Land Survey or Soil Site was established; why a profile was created; or why a site was visited."

related to ANZSoilData/def-au-schema-json#48