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

Fixes: 7df4f9a9 ("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>