What type of Mint should I use?

Pamela -
When a recipe author doesn't denote an ingredient with a particular type - as here with mint - you can usually use anything you prefer or can get in your local markets.
Spearmint is, in fact, the most common mint used in cooking, and should work well.
Here is more info about mint varieties used in cooking - from Martha Stewart a gardening site.
https://www.marthastewart.com/268017/mint-varieties#:~:text=Spearmint%20is%20most%20commonly%20used,course%2C%20mint%20juleps%20and%20mojitos.
https://www.finegardening.com/article/culinary-mint-medley#:~:text=My%20three%20favorites%20for%20cooking,'%2C%20and%20even%20chocolate%20mint.