message: Fix getMockName example of Mock Function document
  According to the API documentation of `getMockName` function ( ), the getMockName function should be called via `mockFunc.getMockName()`, not `mockFunc.mock.getMockName()`.
diff: @@ -280,7 +280,7 @@ expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1][0]).toBe(42);
  // A snapshot will check that a mock was invoked the same number of times,
  // in the same order, with the same arguments. It will also assert on the name.
  expect(mockFunc.mock.calls).toEqual([[arg1, arg2]]);
  -expect(mockFunc.mock.getMockName()).toBe('a mock name');
  +expect(mockFunc.getMockName()).toBe('a mock name');
  ```
  For a complete list of matchers, check out the [reference docs](ExpectAPI.md).
  