doc: platform: cznic: turris-omnia-mcu: Use double backticks for attribute value

Use double backticks instead of quotes for sysfs attribute value.
This makes sphinx generate the "mcu" and "cpu" values in monospace when
rendering to HTML.

Fixes: dfa556e45ae9 ("platform: cznic: turris-omnia-mcu: Add support for MCU connected GPIOs")
Tested-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Marek Behún <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnd Bergmann <[e-mail]>

Documentation/ABI/testing/sysfs-bus-i2c-devices-turris-omnia-mcu

···
 		interrupt.
 
 		This file switches between these two modes:
-		 - "mcu" makes the button press event be handled by the MCU to
+		 - ``mcu`` makes the button press event be handled by the MCU to
 		   change the LEDs panel intensity.
-		 - "cpu" makes the button press event be handled by the CPU.
+		 - ``cpu`` makes the button press event be handled by the CPU.
 
 		Format: %s.