Fix default config: RemoteMirrors -> RemoteMirrorsProber

This was causing an uninitialized constant error.
2 jobs for sh-fix-default-config in 1 minute and 18 seconds (queued for 16 seconds)
Status Job ID Name Coverage
  Test
passed #4476226
rspec

00:00:56

passed #4476227
rubocop

00:01:14