esp32 using incorrect board manager URL

via https://github.com/Arduino-CI/arduino_ci/issues/235#issuecomment-739664613

> The officially recommended [URL for esp32 boards is] in the installation instructions here:
https://github.com/espressif/arduino-esp32/blob/master/docs/arduino-ide/boards_manager.md

And it points to
https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json
