Fix comment in upstairs/src/client.rs

[leftwo]

This comment:

        // downstairs should automatically call PromoteToActive when it reaches
        // the relevant state.

Downstairs don't call, they send a message.

[leftwo]

Also,

Clear this Downstair's ..

The internet says:

Use an apostrophe + s for singular nouns (sea, sky), common nouns ending with s (tigress, mistress), and irregular plural nouns (women, children). Use only the apostrophe for proper nouns ending with s (Tess, Jesus, Texas) and regular plural nouns (cars, protestors).

I believe because the downstairs is being used as a name here, it becomes a proper noun.
So, it should be Downstairs'

This also opens the door to changing downstairs to Downstairs everywhere.
As well as upstairs to Upstairs. go get busy!