🛠️ Makefile: Change deprecated Inkscape flag

This was annoying to figure out because the Inkscape manpage still shows
`--export-png` as a valid option, even though it was removed in the last
release. It has to be `--export-file` now:

    https://wiki.inkscape.org/wiki/index.php/Using_the_Command_Line#New_options

Undocumented features… 👀

Signed-off-by: Justin W. Flory <git@jwf.io>

images/Makefile

@@ -8,7 +8,7 @@ help:
 all: help
 
 %.png: %.svg
-	@inkscape --without-gui --export-area-page --export-width=1890 --export-png=$@ $<
+	@inkscape --without-gui --export-area-page --export-width=1890 --export-file=$@ $<
 
 %.jpg: %.png
 	@convert $< $@