update README.md

Author: long2ice

README.md

@@ -231,6 +231,7 @@ You can use `aerich` out of cli by use `Command` class.
 from aerich import Command
 
 command = Command(tortoise_config=config, app='models')
+await command.init()
 await command.migrate('test')
 ```
 

aerich/__init__.py

@@ -27,7 +27,7 @@ class Command:
     def __init__(
         self,
         tortoise_config: dict,
-        app: str,
+        app: str = "models",
         location: str = "./migrations",
         src_folder: str = ".",
     ):