How do I remove myself as an administrator of a MacBook Air?

See here: link-> What to do before you sell, give away, or trade in your Mac - Apple Support

Certain concerns on Adding wife as Another Admin Account and then removing your Admin Account

The First time the computer was setup as new becomes the defacto Admin Account.

Even after Adding another Admin Account - am pretty sure your wife will not be able to remove the Original Admin Account - that being yours.

Would suggest doing the Before you sell. give away or trade-in suggestion to truly be able to have her have Full Control of both the computer and the former user ( you ) Humour here :-)

Add her as an Admin user. She can then remove any other users.