Update objectscript-class.json

Added a multi-superclass snippet so developers know that () are required around multiple superclasses

Author: isc-solon

snippets/objectscript-class.json

@@ -3,10 +3,14 @@
     "prefix": "Class",
     "body": ["Class $1", "{", "$0", "}"]
   },
-  "Class Definition with super": {
+  "Class Definition with one superclass": {
     "prefix": "Class",
     "body": ["Class $1 Extends ${2|%Persistent,%SerialObject,%RegisteredObject|}", "{", "$0", "}"]
   },
+  "Class Definition with mulitple superclasses": {
+	"prefix": "Class",
+	"body": ["Class $1 Extends (${2|%Persistent,%SerialObject,%RegisteredObject|}, $3)", "{", "$0", "}"]
+  },
   "ClassMethod definition": {
 	"prefix": "ClassMethod",
 	"body": ["ClassMethod $1($2) As ${3:%Status}", "{", "\tset $4 = \\$\\$\\$OK", "\t$0", "\treturn $4","}"]