Sortable Table Example: Separate off-screen portion of caption with p…

…arenthesis instead of comma (pull #3072)

Fixes #3066

Puts off-screen text in parenthesis instead of following a comma.

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Co-authored-by: Matt King <a11yThinker@Gmail.com>
Co-authored-by: Adam Page <adam@adampage.net>

content/patterns/table/examples/sortable-table.html

@@ -65,7 +65,7 @@ <h2 id="ex_label">Example</h2>
           <table class="sortable">
             <caption>
               Students currently enrolled in WAI-ARIA 101
-              <span class="sr-only">, column headers with buttons are sortable.</span>
+              <span class="sr-only">&puncsp;(column headers with buttons are sortable).</span>
             </caption>
             <thead>
               <tr>