Enqueueing a Kernel: add const qualifier (#379)

SYCL 2020 spec section 4.12.1 requires the operator()
in a kernel function object to be const-qualified.

Author: rafbiels

Lesson_Materials/Enqueueing_a_Kernel/Enqueuing_a_Kernel.pdf

@@ -374,7 +374,7 @@
 						<div class="col">
 							<code><pre>
 struct my_kernel { 
-  void operator()(){ 
+  void operator()() const {
     /* kernel function */
   }
 };
@@ -383,6 +383,7 @@
 						<div class="col" data-markdown>
 							* As well as defining SYCL kernels using lambda expressions,
 								You can also define a SYCL kernel using a regular C++ function object.
+							* Define a type with a public const-qualified `operator()` member function.
 						</div>
 					</div>
 				</section>
@@ -395,7 +396,7 @@
 						<div class="col">
 							<code><pre>
 struct my_kernel { 
-  void operator()(){ 
+  void operator()() const {
     /* kernel function */
   }
 };