Avoid using % to format strings, switch to the new method instead.

Run a bitcoin seeder instance for at least 100s in order to get a
dnsseed.dump file. From a build directory:

./src/bitcoin-seeder

(wait 100s then kill it)

mv dnsseed.dump ../contrib/seeds/seeds_main.txt
cd ../contrib/seeds
./makeseeds.py < seeds_main.txt > nodes_main.txt
./generate-seeds.py . > chainparamsseeds.h
cat chainparamsseeds.h

Ensure the chainparamsseeds.h file is correctly formatted.