docs: fix typo (#556)

Author: wjx0820

docs/guide/mocking.md

@@ -59,7 +59,7 @@ describe('purchasing flow', () => {
 
 Mocking functions can be split up into two different categories; *spying & mocking*.
 
-Sometimes all you need is to validate wether or not a specific function has been called (and possibly which arguments were passed). In these cases a spy would be all we need which you can use directly with `vi.spyOn()` ([read more here](/api/#vi-spyon)).
+Sometimes all you need is to validate whether or not a specific function has been called (and possibly which arguments were passed). In these cases a spy would be all we need which you can use directly with `vi.spyOn()` ([read more here](/api/#vi-spyon)).
 
 However spies can only help you **spy** on functions, they are not able to alter the implementation of those functions. In the case where we do need to create a fake (or mocked) version of a function we can  use `vi.fn()` ([read more here](/api/#vi-fn)).