Fixup code blocks

Summary:
This patch uses the backtick-style code block consistently throughout the site
and removes errant leading spaces that render poorly on small screens when code
lines happen to wrap.

Test Plan:
make serve
Navigate to http://localhost:4000/cashaddr/ and verify it still renders well.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7194