Fix bunchee command execution in readme

[piotr-cz]

Executing with npm results in

npm bunchee --prepare

Unknown command: "bunchee"

To see a list of supported npm commands, run:
  npm help

[huozhi]

That one is by purpose for the installed case. there's another line below with npx case

# "Or use with npx"
# npx bunchee@latest --prepare

[piotr-cz]

@huozhi

Command fails when bunchee is installed.

To reproduce, try:

npm install bunchee
npm bunchee --prepare

[huozhi]

oh that works with pnpm, for npm I guess we need to do npm exec bunchee --prepare?

[piotr-cz]

@huozhi Yes, for npm works both npm exec bunchee --prepare and npx bunchee --prepare