bsp-guide: update lonely "4.12" kernel reference to "6.12"

To accompany earlier updating of kernel version, update this lonely
reference to be consistent.

(From yocto-docs rev: 97cd3ee7f3bf1de8454708d1852ea9cdbd45c39b)

Signed-off-by: "Robert P. J. Day" <rpjday@crashcourse.ca>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit e26c2018cd663de91ee08e0cba55eda1a4c30210)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/bsp-guide/bsp.rst

@@ -1184,7 +1184,7 @@ Use these steps to create a BSP layer:
 
 -  *Create a Kernel Recipe:* Create a kernel recipe in
    ``recipes-kernel/linux`` by either using a kernel append file or a
-   new custom kernel recipe file (e.g. ``linux-yocto_4.12.bb``). The BSP
+   new custom kernel recipe file (e.g. ``linux-yocto_6.12.bb``). The BSP
    layers mentioned in the previous step also contain different kernel
    examples. See the ":ref:`kernel-dev/common:modifying an existing recipe`"
    section in the Yocto Project Linux Kernel Development Manual for