fix: correct entity field syntax in README code example (#10)

Fields Id and String require parentheses when used as class attributes.
Without `Id()` and `String()`, instantiating the entity with keyword
arguments raises a TypeError at runtime.

Fixes #10

Author: ElizioMartins

README.md

@@ -47,8 +47,8 @@ from objectbox import Entity, Id, Store, String
 
 @Entity()
 class Person:
-    id = Id
-    name = String
+    id = Id()
+    name = String()
 
 # The ObjectBox Store represents a database; keep it around...
 store = Store()