ledtrig-activity: Grammar s/a immediate/an immediate/

Fixes: 7df4f9a ("leds: ledtrig-activity: Add a system activity LED trigger")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>

drivers/leds/trigger/Kconfig

@@ -81,7 +81,7 @@ config LEDS_TRIGGER_ACTIVITY
 	tristate "LED activity Trigger"
 	depends on LEDS_TRIGGERS
 	help
-	  This allows LEDs to be controlled by a immediate CPU usage.
+	  This allows LEDs to be controlled by an immediate CPU usage.
 	  The flash frequency and duty cycle varies from faint flashes to
 	  intense brightness depending on the instant CPU load.
 	  If unsure, say N.