docs: fix clone URL and directory in README setup instructions

[mateuscgoncalves]

Problem

If we follow the README steps as written:

git clone https://github.com/libpdf/core.git
cd libpdf

We face two issues:

1. The clone fails because the repository is under the organization LibPDF-js, not libpdf:

$ git clone https://github.com/libpdf/core.git
Cloning into 'core'...
remote: Repository not found.
fatal: repository 'https://github.com/libpdf/core.git/' not found

2. Even with the correct URL, git clone creates a core/ directory, so cd libpdf fails as well.

Fix

-git clone https://github.com/libpdf/core.git
-cd libpdf
+git clone https://github.com/LibPDF-js/core.git
+cd core

This is the first command a new contributor runs, so it currently blocks onboarding and contradicts Development Setup section in CONTRIBUTING.md, which already uses LibPDF-js/core and cd core.

Note: this README is also on npm, so the fix will propagate there on the next release

[vercel]

@mateuscgoncalves is attempting to deploy a commit to the mythie's projects Team on Vercel.

A member of the Team first needs to authorize it.