SW-44: fixed typo

SAS/ResourceAssignment/ResourceAssignmentDatabase/radb/sql/add_functions_and_triggers.sql

@@ -609,7 +609,7 @@ BEGIN
         SELECT * from clock_timestamp() into proc_end;
         RAISE NOTICE 'get_resource_claimable_capacity_between_b took %', proc_end - proc_start;
 
-        IF current_claimed_usage IS NULL THEN
+        IF current_claimed_usage IS NOT NULL THEN
             RETURN available_capacity + current_claimed_usage - max_resource_usage_value;
         END IF;