[9.x] Fix Collection::combine return docblock

[Josh-G]

Previously, the combine docblock always used the template type of the first collections keys. It should be TValue since we're combining the values of both collections.

Added a type assertion as well, without this change it fails with

  :605   Expected type Illuminate\Support\Collection<string, string>, actual: Illuminate\Support\Collection<int, string>  

[nunomaduro]

Thank you!