xen/arm: optee: update description in Kconfig

OP-TEE mediator now is "Tech Preview" state, and we want to update
it's description in Kconfig accordingly.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/arch/arm/tee/Kconfig

@@ -3,7 +3,8 @@ config OPTEE
 	default n
 	depends on TEE
 	help
-	  Enable experimental OP-TEE mediator. It allows guests to access
-	  OP-TEE running on your platform. This requires virtualization-enabled
-	  OP-TEE present. You can learn more about virtualization for OP-TEE
-	  at https://optee.readthedocs.io/architecture/virtualization.html
+	  Enable the OP-TEE mediator. It allows guests to access
+	  OP-TEE running on your platform. This requires
+	  virtualization-enabled OP-TEE present. You can learn more
+	  about virtualization for OP-TEE at
+	  https://optee.readthedocs.io/architecture/virtualization.html