Fixed bug I introduced with register_function()

sqlite_utils/db.py

@@ -264,7 +264,7 @@ class Database:
         return "<Database {}>".format(self.conn)
 
     def register_function(
-        self, fn: Callable, deterministic: bool = False, replace: bool = False
+        self, fn: Callable=None, deterministic: bool = False, replace: bool = False
     ):
         """
         ``fn`` will be made available as a function within SQL, with the same name and number
@@ -274,6 +274,12 @@ class Database:
             def upper(value):
                 return str(value).upper()
 
+        The decorator can take arguments::
+
+            @db.register(deterministic=True, replace=True)
+            def upper(value):
+                return str(value).upper()
+
         - ``deterministic`` - set ``True`` for functions that always returns the same output for a given input
         - ``replace`` - set ``True`` to replace an existing function with the same name - otherwise throw an error
         """