[SE-0396] Correct description for Either type with Codable generics

[Jager-yoo]

Hi, I corrected the description for Either with Codable generics.
Please review the changes. Thanks!

[allevato]

For what it's worth, the original text is also grammatically correct; and having the condition (the "where" clause) precede the thing that's applied based on that condition ("are Codable") discloses the information in clearer order, in my opinion.

[Jager-yoo]

I see, makes sense! Thank you for clarifying.