Minor, fix old copy-paste error in comment

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/signal.c

@@ -34,7 +34,7 @@
  * Create a signal watcher
  * @param ctx    A valid libuEv context
  * @param w      Pointer to an uev_t watcher
- * @param cb     Timer callback
+ * @param cb     Signal callback
  * @param arg    Optional callback argument
  * @param signo  Signal to watch for
  *