Fix command to install magicbook

[narisada014]

Description

When trying to build reading the README, I could't execute magicbook.
I was able to execute magicbook by adding the -g option to the install command.

[chiphuyen]

Hi @narisada014,

Thanks for your contribution. The -g option is to install the package globally. Some users might not want that. I'll leave the PR here so people who have the same problem can refer to it!

[jsdps]

Faced a couple issues, most probably versioning issue
So listing them here, in case someone reaches this page

• node/ magicbook installation issue
  can get a bunch of issues like python version, python path (on Mac)
  Fix: avoid using Brew to install node as it installs latest version (15.x currently which gives a bunch of error with Prince, a dependency). Instead download installer from node's website and then install (use stable release, avoid latest)

• magicbook not found
  even after successful node and magicbook installation, this error can show
  Fix: use the -g flag as mentioned in this PR. Otherwise it cannot recognise the command