Excerpt from this article:
Like humans, Apple’s virtual assistant can sometimes stumble over names that don’t read the same way they sound when spoken aloud. But as with humans, you can tell Siri the proper pronunciation of the name for future reference.
The next time Siri mangles a name, tap the microphone button and say, “That’s not how you pronounce [Name].” The program should respond with, “O.K., how do you pronounce the name [Name]?” Say the correct pronunciation of your first and last name as clearly as you can.
Siri will then fire back with, “O.K., thank you. Which pronunciation should I use?” and offer a few variations of your first name to play back. After you have listened to the choices, tap Select next to the one you want and then move on to Siri’s attempts to pronounce your last name.