fixup: docs

Author: mcous

README.md

@@ -271,7 +271,7 @@ By default, if arguments do not match, a vitest-when stub will no-op and return
 ```ts
 const spy = vi.fn().mockReturnValue('you messed up!')
 
-subject.when(spy).calledWith('hello').thenReturn('world')
+when(spy).calledWith('hello').thenReturn('world')
 
 spy('hello') // "world"
 spy('jello') // "you messed up!"