fix(typescript/trpc-nextjs): typo "singupUser" instead of "signupUser"

[Arshdeep Singh]

Hey there 👋 I recently tried to test the trpc-nextjs - Prisma example and I noticed that the signupUser mutation is mis-spelled as singupUser.

I decided to correct the spelling which took place in the following files:

• typescript/trpc-nextjs/src/pages/signup.tsx
• typescript/trpc-nextjs/src/server/api/routers/user.ts

but if you think it's the other way around please let me know.

Hope this helps!