Links to book's index are broken

[timagixe]

Describe the bug
Both links in the repository's README.md file that should navigate user to a book's index are broken.

Expected alternatives
Update links to a valid one from (i.e. for English Book/content/en/Index.md)

Screenshots
N/A

Add references
N/A

[tshemsedinov]

@timagixe Thanks!