Fix a typo (duplicated word) in Documentation/leds/leds-class.txt

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Richard Purdie <rchard.purdie@linuxfoundation.org>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>