distro: rename ListArchs to ListArches

In PR#395 we discussed the spelling of archs vs. arches and we agreed to
use arches. This patch only renames the public method `ListArchs`in the
`Distro` interface.

internal/distro/test/distro.go

@@ -16,7 +16,7 @@ type testImageType struct{}
 const name = "test-distro"
 const modulePlatformID = "platform:test"
 
-func (d *TestDistro) ListArchs() []string {
+func (d *TestDistro) ListArches() []string {
 	return []string{"test_arch"}
 }