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/chip.c b/kernel/irq/chip.c
index 686be4b..4188a0a 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -7,7 +7,7 @@
  * This file contains the core interrupt handling code, for irq-chip
  * based architectures.
  *
- * Detailed information is available in Documentation/DocBook/genericirq
+ * Detailed information is available in Documentation/core-api/genericirq.rst
  */
 
 #include <linux/irq.h>