Fix bad advice in cook book (#2308)

This line directly contradicts the warning that google mock spits out on unused mock calls:
```
NOTE: You can safely ignore the above warning unless this call should not happen.  Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call.  See https://github.com/google/googletest/blob/master/googlemock/docs/CookBook.md#knowing-when-to-expect for details.
```
One or the other should be changed, and I believe the advice in this file is incorrect.