opensubtitles: Fix invalid format specifier

As explainer in !319 (merged), we get the error

Traceback (most recent call last):
  File "/app/lib/totem/plugins/opensubtitles/opensubtitles.py", line 789, in __on_find_clicked
    (movie_hash, movie_size) = hash_file (self._filename)
  File "/app/lib/totem/plugins/opensubtitles/hash.py", line 42, in hash_file
    returnedhash = f"{file_hash:%016x}"
ValueError: Invalid format specifier

which prevent us from downloading subtitles

Thanks