[doc] Fix formatting in init.md

Summary:
D14899 introduced a random mid-sentence blank line.
Fix that, and also shorten some excessively long lines to 80 chars while we're at it

Test Plan: Look at the file, see that the formatting looks better

Reviewers: alitayin, #bitcoin_abc, Fabien, bytesofman

Reviewed By: #bitcoin_abc, Fabien, bytesofman

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14960