message: Update developer-guide.md
  Corrected Ubuntu toolchain installation guide as described in:
diff: @@ -52,8 +52,9 @@ $ python --version # should be 3.7 or later
  #### Linux (Ubuntu)
  ```bash
  -sudo apt-get install gcc-arm-none-eabi
  -sudo apt-get install gdb-arm-none-eabi
  +sudo add-apt-repository ppa:team-gcc-arm-embedded/ppa
  +sudo apt-get update
  +sudo apt-get install gcc-arm-embedded
  sudo apt-get install openocd
  sudo add-apt-repository ppa:jonathonf/tup && sudo apt-get update && sudo apt-get install tup
  ```
  