overlayfs.rst: fix ReST formatting

Fix some indentation issues and fix missing newlines in quoted text
by converting quoted text to code blocks.

Reported-by: Christian Brauner <brauner@kernel.org>
Suggested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>