Update README.md

Use https:// for board manager URL.

README.md

@@ -13,7 +13,7 @@ Program [Nordic Semiconductor](https://www.nordicsemi.com) nR5 based boards with
  1. [Download and install the Arduino IDE](https://www.arduino.cc/en/Main/Software)
  2. Start the Arduino IDE
  3. Go into Preferences
- 4. Add ```http://sandeepmistry.github.io/arduino-nRF5/package_nRF5_boards_index.json``` as an "Additional Board Manager URL"
+ 4. Add ```https://sandeepmistry.github.io/arduino-nRF5/package_nRF5_boards_index.json``` as an "Additional Board Manager URL"
  5. Open the Boards Manager from the Tools -> Board menu and install "Nordic Semiconductor nRF5 Boards"
  6. Select your nRF5 board from the Tools -> Board menu