docs: update loopback types link

Author: Agnes Lin

README.md

@@ -473,7 +473,8 @@ export class User extends Entity {
 
 ### Type mapping
 
-See [LoopBack types](http://loopback.io/doc/en/lb3/LoopBack-types.html) for details on LoopBack's data types.
+See [LoopBack 4 types](http://loopback.io/doc/en/lb4/LoopBack-types.html) (or [LoopBack 3 types](http://loopback.io/doc/en/lb3/LoopBack-types.html)) for
+details on LoopBack's data types.
 
 #### JSON to Oracle Types