Update error class description

spark/src/main/resources/error/delta-error-classes.json

@@ -1648,7 +1648,7 @@
   },
   "DELTA_NESTED_NOT_NULL_CONSTRAINT" : {
     "message" : [
-      "The <nestType> type of the field <parent> contains a NOT NULL constraint. Delta does not support NOT NULL constraints nested within arrays or maps. To suppress this error and silently ignore the specified constraints, set <configKey> = true.",
+      "The <nestType> type of the field <parent> contains a NOT NULL constraint. Delta does not support NOT NULL constraints nested within arrays or maps or within fields nested inside nullable structs. To suppress this error and silently ignore the specified constraints, set <configKey> = true.",
       "Parsed <nestType> type:",
       "<nestedPrettyJson>"
     ],