lib: update location of kgdb documentation

The documentation was moved from DocBook directory.
Update its location.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

lib/Kconfig.kgdb

@@ -13,7 +13,7 @@ menuconfig KGDB
 	  CONFIG_FRAME_POINTER to aid in producing more reliable stack
 	  backtraces in the external debugger.  Documentation of
 	  kernel debugger is available at http://kgdb.sourceforge.net
-	  as well as in DocBook form in Documentation/DocBook/.  If
+	  as well as in Documentation/dev-tools/kgdb.rst.  If
 	  unsure, say N.
 
 if KGDB