xar: add tokens

Made the filename consistent with other pages.

pages/linux/xar.md

@@ -4,12 +4,12 @@
 
 - Create a xar archive of all files in {{directory}}:
 
-`xar -cf sample.xar {{directory}}`
+`xar -cf {{archive.xar}} {{path/to/directory}}`
 
 - List the contents of the xar archive sample.xar:
 
-`xar -tf sample.xar`
+`xar -tf {{archive.xar}}`
 
 - Extract the contents of sample.xar to the current working directory:
 
-`xar -xf sample.xar`
+`xar -xf {{archive.xar}}`