Fix documentation for ISO verification on Windows

Originally created by Tails on #6082 (Redmine)

MD5 Rebound Hasher does not work with recent Firefox and its development is dead.

Upstream tickets:

• http://sourceforge.net/p/md5rehasher/bugs/8/
• https://bugzilla.mozilla.org/show_bug.cgi?id=861684

We need to find alternative solutions to check SHA256 sums.

Related issues

• Is duplicate of #6851 (closed)