`epiprocess` vignette "other key" example should use county as `geo_value` and state as additional key

[nmdefries]

The "other keys" example uses county_code as the additional key and state as the geo_value. Probably we want geo_value to fully specify the geographical area of an observation. These should be swapped so that county FIPS codes are used for the geo_value field.

[nmdefries]

Also appears as an example in as_epi_df documentation.