trivial: fix an -> a typos in documentation and comments

It is always "an" if there is a vowel _spoken_ (not written).
So it is:
"an hour" (spoken vowel)
but
"a uniform" (spoken 'j')

Signed-off-by: Frederik Schwarzer <schwarzerf@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/macintosh/Kconfig b/drivers/macintosh/Kconfig
index b526596..173cf55 100644
--- a/drivers/macintosh/Kconfig
+++ b/drivers/macintosh/Kconfig
@@ -138,7 +138,7 @@
 	  Say Y here to enable Macintosh specific extensions of the generic
 	  backlight code. With this enabled, the brightness keys on older
 	  PowerBooks will be enabled so you can change the screen brightness.
-	  Newer models should use an userspace daemon like pbbuttonsd.
+	  Newer models should use a userspace daemon like pbbuttonsd.
 
 config PMAC_BACKLIGHT_LEGACY
 	bool "Provide legacy ioctl's on /dev/pmu for the backlight"