[doc]fix error example

[Tankunpeng]

const total = wrapper.find(Bar).reduce((amount, n) => amount + n.prop('amount'));

In this example, total is an Object, because of there is no initial reduce value. we must set the initial value with 0 to get the right result.
And, the right result is 14.

[ljharb]

Thanks, good catch. Can you confirm the mount docs for reduce don’t need the same fix?

[Tankunpeng]

Sorry, the mount docs also need fix. I start a new pull request. #2066

[ljharb]

Thanks!