Add an example of updateMask

[simonebracaloni]

Problem

I spent a lot of time figuring out how to pass updateMask in plain JS

Solution

It would be very useful to update the documentation of updateJobs with a filled updateMask example like:

const job = {
        name,
        description,
        schedule
    };
const updateMask = {
    paths: ["description","schedule"]
};
const request = {
    job,
    updateMask
};

Alternative solution

As this actually applies to all the JS libraries backed by grpc this documentation may be added somewhere else.

[averikitsch]

Fixed in #265