tools/gpio: fix gpio-event-mon header comment

Fixes: 97f69747 ('tools/gpio: add the gpio-event-mon tool')
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

tools/gpio/gpio-event-mon.c

 /*
- * gpio-hammer - example swiss army knife to shake GPIO lines on a system
+ * gpio-event-mon - monitor GPIO line events from userspace
  *
  * Copyright (C) 2016 Linus Walleij
  *