font-patcher: Fix debug format string

[why]
The format string is invalid and results in a runtime error if that
particular warning is issued (which is only the case for Terminus).

[how]
A verbatim percent sign must be encoded as '%%'.

[note]
This has already been fixed before with commit
  f81564f  font-patcher: Fix typo in logger output

but the change has been lost by rebasing :-(

See also #1350

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -1154,7 +1154,7 @@ class font_patcher:
             if our_btb == hhea_btb:
                 metrics = Metric.TYPO if use_typo else Metric.WIN # conforming font
             elif abs(our_btb - hhea_btb) / our_btb < 0.03:
-                logger.info("Font vertical metrics slightly off (%.1f%)", (our_btb - hhea_btb) / our_btb * 100.0)
+                logger.info("Font vertical metrics slightly off (%.1f%%)", (our_btb - hhea_btb) / our_btb * 100.0)
                 metrics = Metric.TYPO if use_typo else Metric.WIN
             else:
                 # Try the other metric