[Spec] `perSlotSizeQueryParam` isn't initialized

[caraitto]

In the spec, perSlotSizeQueryParam never gets initialized.

Also, this line looks suspect: https://wicg.github.io/turtledove/#:~:text=Let%20perSlotSizeQueryParam%20be%20perBuyerGenerator%5BsignalsUrl

It's assigning a dictionary to another dictionary with different types. (That wouldn't compile if this were C++)

[caraitto]

Here's a permalink to the suspect line in source:

turtledove/spec.bs

Line 1422 in c69e08c

1. Let |perSlotSizeQueryParam| be |perBuyerGenerator|[|signalsUrl|].

[caraitto]

@MattMenke2