[Enhancement]: Add MFA details to Ruby example

[ford-at-aws]

Background story

There is a code library's MFA page has a Ruby entry, but that Ruby entry does not contain any MFA steps (instead, it authenticates without MFA).

What does this example accomplish?

Robustness!

Which AWS service(s)?

STS

Which AWS SDKs or tools?

• All languages
• .NET
• C++
• Go (v2)
• Java
• Java (v2)
• JavaScript
• JavaScript (v3)
• Kotlin
• PHP
• Python
• Ruby
• Rust
• Swift
• Not applicable

Are there existing code examples to leverage?

https://docs.aws.amazon.com/en_us/IAM/latest/UserGuide/id_credentials_mfa_sample-code.html

Do you have any reference code?

https://docs.aws.amazon.com/sdk-for-ruby/v3/api/Aws/STS/Client.html#assume_role-instance_method