Invalid link in ebook generating

Hi,

First, thanks a lot for sharing the linux knowledge in this book.

I'm reading through the README file and it looks like the link

**linux-insides** --Generating eBooks and PDFs - [documentation](https://github.com/GitbookIO/gitbook/blob/master/docs/ebook.md)

here is no longer valid.

The issue points to a broken link in the README file. Locate the README.md in the repository root, find the line referencing 'https://github.com/GitbookIO/gitbook/blob/master/docs/ebook.md', and replace it with a working link to GitBook's ebook documentation or an alternative resource. Verify the change by checking the rendered README.

Written by the indexing model from the issue text.