apacheGH-38847: [Documentation][C++] Explicitly note that compute is …

…optional

docs/source/cpp/tutorials/compute_tutorial.rst

@@ -41,6 +41,9 @@ Before continuing, make sure you have:
 
 1. An Arrow installation, which you can set up here: :doc:`/cpp/build_system`
 
+  - Be sure that your arrow installation
+    :doc:`includes the compute kernels. </cpp/building.html#optional-components>`
+
 2. An understanding of basic Arrow data structures from :doc:`/cpp/tutorials/basic_arrow`
 
 Setup
@@ -50,7 +53,7 @@ Before running some computations, we need to fill in a couple gaps:
 
 1. We need to include necessary headers.
 
-2. ``A main()`` is needed to glue things together.
+2. A ``main()`` is needed to glue things together.
 
 3. We need data to play with.