uefi: add missing f-string prefix in _find_bootentry() error

Add missing f-string prefix for proper string interpolation in
the error message of _find_bootentry()

src/utils/uefi.py

@@ -25,7 +25,7 @@ def _find_bootentry(entries, label):
         if entry['description'] == label:
             return entry
     else:
-        raise OgError('Boot entry {label} not found')
+        raise OgError(f'Boot entry {label} not found')
 
 
 def _strip_boot_prefix(entry):