fix formatting of code sample

correcting from one backtick to two backticks

as documented here https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#inline-markup

Signed-off-by: Alexander Müller <28591861+alexanderdd@users.noreply.github.com>

Author: alexanderdd

admin_manual/occ_command.rst

@@ -250,7 +250,7 @@ enabled or disabled::
 
  sudo -E -u www-data php occ app:list
 
-List all of your installed and enabled (flag `--enabled`) or disabled (flag `--disabled`) apps::
+List all of your installed and enabled (flag ``--enabled``) or disabled (flag ``--disabled``) apps::
 
  sudo -E -u www-data php occ app:list --enabled