sys-utils: fix SELinux context example in mount.8

sys-utils: fix SELinux context example in mount.8

In the description of the context option, the example which shows how to
properly quote is displayed incorrectly on terminals > 80 columns.  This
leaves a seemingly needless '\' in the command, e.g.:

    mount -t tmpfs none /mnt -o \ 'context="system_u:...'

The intent is to display the command properly on terminals <= 80
columns.  Use a literal block to ensure the code is displayed
consistently, regardless of the terminal width.

Connect the blockquote to the previous indented items in the context
option description to ensure it is properly indented.

Signed-off-by: Todd Zullinger <tmz@pobox.com>