comparators/directory.py: fix whitespace for black 21.12

On black-21.12 test fails as:

    --- diffoscope/comparators/directory.py     2022-01-26 11:54:10.953755 +0000
    +++ diffoscope/comparators/directory.py     2022-01-26 11:54:18.335859 +0000
    @@ -46,11 +46,10 @@
                     "%Y-%m-%d %H:%M:%S",
                     "-f",
                     "%Sp %l %Su %Sg %z %Sm %k %b %#Xf",
                     self.path,
                 ]
    -
     else:
         class Stat(Command):

The change drops extra whitespace.