Fix links in PE upgrade instructions.
Fix links in PE upgrade instructions.

volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io,thingsboard/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io