doc: polling: Fix semaphore contested description
doc: polling: Fix semaphore contested description

If a semaphore is contested, it is possible that the semaphore will no
longer be available when k_sem_take() is called, not k_sem_give().

Fix few typos and explicitly mention poll events instead of "they".

Signed-off-by: Tomasz Moń <f44a0a0857de22882044c93edbe6b590cb840e13@nordicsemi.no>

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