linux-yocto-custom: Fix comment override syntax

The override format in a comment is incorrect, fix it.

(From OE-Core rev: 7249f532efee968522a2d0a3fe02c9f2dd7c03fe)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit df5875cb22585f10a2e8782c0534ae4625e85bb0)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb

@@ -7,7 +7,7 @@ SUMMARY = "An example kernel recipe that uses the linux-yocto and oe-core"
 #   To use linux-yocto-custom in your layer, copy this recipe (optionally
 #   rename it as well) and modify it appropriately for your machine. i.e.:
 #
-#     COMPATIBLE_MACHINE_yourmachine = "yourmachine"
+#     COMPATIBLE_MACHINE:yourmachine = "yourmachine"
 #
 #   You must also provide a Linux kernel configuration. The most direct
 #   method is to copy your .config to files/defconfig in your layer,