doc: fix references to driver-model

Fix some errors pointed out by 'make refcheckdocs'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/i2c/sh_i2c.c b/drivers/i2c/sh_i2c.c
index b69d213..834f1f2 100644
--- a/drivers/i2c/sh_i2c.c
+++ b/drivers/i2c/sh_i2c.c
@@ -4,7 +4,7 @@
  * Copyright (C) 2011, 2013 Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
  *
  * NOTE: This driver should be converted to driver model before June 2017.
- * Please see doc/driver-model/i2c-howto.txt for instructions.
+ * Please see doc/driver-model/i2c-howto.rst for instructions.
  */
 
 #include <common.h>