irq: update genericirq book location

This book got converted from DocBook. Update its references to
point to the current location.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index d3f2490..bbf9a71 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -6,7 +6,7 @@
  *
  * This file contains the core interrupt handling code.
  *
- * Detailed information is available in Documentation/DocBook/genericirq
+ * Detailed information is available in Documentation/core-api/genericirq.rst
  *
  */