Update OpenWrt 18.06 wiki page to RC2

Now that OpenWrt 18.06 RC2 has been released, could an admin please update the wiki page accordingly? I propose the following changes:

  • In the "Upcoming Stable Release" section, for the download URL and its description, change RC1 to RC2. The "expected release date" lies in the past and should also be updated or deleted.
  • In the Roadmap section, delete the first two entries, RC1 and RC2. They already have copies in the History section, where they belong.