Update download link to the offline version of the handbook

[ntluong95]

The function epirhandbook::download_book() need to be updated to:

• Update the link to download offline copy across language version: https://github.com/appliedepi/epiRhandbook_eng/raw/master/offline_long/index..html ( is the language identifier)
• Add an argument to specify which language to be download, for example download_book(fr)