boards: rpi_pico: Fix doc typos and grammar
boards: rpi_pico: Fix doc typos and grammar

Fix a few typos and grammar mistakes in the rpi_pico documentation.

Signed-off-by: Yonatan Schachter <c03ea33f3e5919b7d64cfd097e68ce03a1b1a328@gmail.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr