.formatter.exs: Format optional migrations

(There are no changes to optional migrations since they were manually
formatted in !3207)

.formatter.exs

 [
-  inputs: ["mix.exs", "{config,lib,test}/**/*.{ex,exs}", "priv/repo/migrations/*.exs", "priv/scrubbers/*.ex"]
+  inputs: ["mix.exs", "{config,lib,test}/**/*.{ex,exs}", "priv/repo/migrations/*.exs", "priv/repo/optional_migrations/**/*.exs", "priv/scrubbers/*.ex"]
 ]