[Backport] Fix Markdown formatting issues in init.md

Summary:
The section headers in in init.md were not correct markdown syntax.

Backport or Bitcoin Core PR#10878

Test Plan: Visual check that the section titles are now correctly rendered in pycharm and github.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7483