Make tar page consistent

It might confuse users if everything has no dash in front but the last command does. tar doesn't need the dash so let's keep it that way.

pages/common/tar.md

@@ -29,4 +29,4 @@
 
 - list the contents of a tar file
 
-`tar -tvf {{source.tar}}`
+`tar tvf {{source.tar}}`