fix: use pnpm in readme

[jwstanly]

Changes Made

1. Updated README.md to use pnpm commands instead of npm commands to match the existing pnpm-lock.yaml file in the repository.
2. Simplified the "dev" script in package.json to use the standard Next.js command format.

Further Potential Changes

I also noticed the pnpm-lock.yaml is version 6. Running pnpm install using pnpm version 9.12.3 locally changes the pnpm-lock.yaml version number to 9 as well as modifies the lock file contents. If this repo wants to suggest using pnpm it might be nice to specify a version of pnpm or update the lock file. But also totally get maybe this repo wants to actually encourage npm use or be agnositc

Additional Note

I came across this when debugging an issue with HeyGen's StreamingAvatarSDK and opened an issue there: HeyGen-Official/StreamingAvatarSDK#57