boot/optee-os: fix typo in help message of BR2_TARGET_OPTEE_OS_CUSTOM_TARBALL

The custom tarball option should be given a URL pointing to an OP-TEE OS
tarball and not a "kernel" one.

Fixes: 9c79b369d69c "boot/optee-os: add support for custom tarball URL"
Cc: Quentin Schulz <foss+buildroot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

boot/optee-os/Config.in

@@ -28,7 +28,7 @@ config BR2_TARGET_OPTEE_OS_LATEST
 config BR2_TARGET_OPTEE_OS_CUSTOM_TARBALL
 	bool "Custom tarball"
 	help
-	  This option allows to specify a URL pointing to a kernel
+	  This option allows to specify a URL pointing to an OP-TEE OS
 	  source tarball. This URL can use any protocol recognized by
 	  Buildroot, like http://, ftp://, file:// or scp://.