Use ".Sy root" instead of ".Em root" when talking about the root user.

Replace MANDOCPROG with "mandoc" now that MANDOCPROG has been removed.

docs/sudo_plugin_python.mdoc.in

@@ -1535,14 +1535,14 @@ will not load the Python interpreter or the Python libraries.
 .Pp
 By default, a Python plugin can only import Python modules which are
 owned by
-.Em root
+.Sy root
 and are only writable by the owner.
 The reason for this is to prevent a file getting imported accidentally
 which is modifiable by a non-root user.
 As
 .Nm sudo
 plugins run as
-.Em root ,
+.Sy root ,
 accidentally importing such file would make it possible for any user
 (having write access) to execute any code with administrative rights.
 .Pp