Rewrite Rules: Use double quotes instead of backticks in `.htaccess`...

Rewrite Rules: Use double quotes instead of backticks in `.htaccess` instructions about `# BEGIN/END` markers.

Previously, the backticks resulted in extra markup being added when posted on support forums.

Props mayankmajeji, knutsp.
Fixes #49868.
Built from https://develop.svn.wordpress.org/trunk@47571


git-svn-id: https://core.svn.wordpress.org/trunk@47346 1a063a9b-81f0-0310-95a4-ce76da25c4cd