Update test for H71.html (#3096)

Modify test section to include the requirement that legend is first
child in fieldset, as is technically required

Closes  #3090

---------

Co-authored-by: Francis Storr <francis.storr@intel.com>
Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

techniques/html/H71.html

@@ -112,7 +112,7 @@ <h3>Procedure</h3>
          <p>For groups of related controls where the individual labels for each control do not provide a sufficient description, and an additional group level description is needed:</p>
          <ol>
             <li>Check that the group of logically related <code class="language-html">input</code> or <code class="language-html">select</code> elements are contained within <code class="language-html">fieldset</code> elements.</li>
-            <li>Check that each <code class="language-html">fieldset</code> has a <code class="language-html">legend</code> element that includes a description for that group.</li>
+            <li>Check that each <code class="language-html">fieldset</code> has a <code class="language-html">legend</code> element that is the first child in the <code class="language-html">fieldset</code> and includes a description for that group.</li>
          </ol>
       </section>
       <section class="results">