boards/nrf52840dongle doc: Fix heading level

boards/nrf52840dongle/doc.txt

@@ -3,15 +3,15 @@
 @ingroup     boards
 @brief       Support for the nRF52840-Dongle
 
-### Quick start
+## Quick start
 
 - Plug into a USB port.
 - `$ make BOARD=nrf52840dongle -C examples/saul flash term`
   - See [Flash the board](@ref nrf52840dongle_flash) if anything goes wrong.
 - `> saul write 2 10 40 10`
   - The LED glows in a soft turquise.
 
-### General information
+## General information
 
 The nRF52840-Dongle is a bare USB-stick shaped device that houses barely
 anything than the nRF52840 itself, which offers BLE and 802.15.4 and USB
@@ -24,12 +24,12 @@ built-in bootloader.
 The board features two LEDs (LD1: green, LD2: RGB), a user (SW1) and a
 reset button as well as 15 configurable external pins.
 
-### Links
+## Links
 
 - [nRF52840 web page](https://www.nordicsemi.com/?sc_itemid=%7BCDCCA013-FE4C-4655-B20C-1557AB6568C9%7D)
 - [documentation and hardware description](https://infocenter.nordicsemi.com/topic/ug_nrf52840_dongle/UG/nrf52840_Dongle/intro.html?cp=3_0_5)
 
-### Flash the board {#nrf52840dongle_flash}
+## Flash the board {#nrf52840dongle_flash}
 
 The board is flashed using its on-board boot loader; the
 [nrfutil](https://github.com/NordicSemiconductor/pc-nrfutil) program needs to
@@ -45,14 +45,14 @@ manually by pressing the board's reset button.
 
 Readiness of the bootloader is indicated by LD2 pulsing in red.
 
-#### nrfutil installation
+### nrfutil installation
 
 On systems with Python 2 available, `pip install nrfutil` works.
 
 On systems with Python 3, a recent version of pip is required to install all dependencies;
 you may need to run `pip install --upgrade pip` before being able to run `pip install nrfutil` successfully.
 
-### Accessing STDIO
+## Accessing STDIO
 
 The usual way to obtain a console on this board is using an emulated USB serial port (CDC-ACM).
 This is available automatically using the `stdio_cdc_acm` module,