[displaymanager] Fix Python errors

Apparently nobody ever hit the else-branch here (because
each DM has exactly one implementation -- that's what the
check is there for!) because the logging of the error
itself would raise IndexError or ValueError.

src/modules/displaymanager/main.py

@@ -923,7 +923,7 @@ def run():
             else:
                 dm_instance = None
         else:
-            libcalamares.utils.debug("{!s} has {!d} implementation classes.".format(dm).format(len(impl)))
+            libcalamares.utils.debug("{!s} has {!s} implementation classes.".format(dm, len(impl)))
 
         if dm_instance is None:
             libcalamares.utils.debug("{!s} selected but not installed".format(dm))