Correct id of img element in aria-labelledby test (#45854)

This test includes an aria-labelledby reference to id "nested_image_label3"
but that id doesn't exist. Instead, "nested_image_label_3" exists. This commit
updates the id, removing the underscore to fit the pattern of the test file.

Author: nmlapre

accname/name/comp_name_from_content.html

@@ -239,7 +239,7 @@ <h3 data-expectedlabel="image link" data-testname="heading with link referencing
     <span class="note" id="crossref_link">link</span><!-- this text is skipped the first time around because of aria-labelledby on parent element -->
   </a>
   <!-- but it's picked up again in inverse order b/c of cross-referencial aria-labelledby edge case -->
-  <img id="nested_image_label_3" alt="image" aria-labelledby="crossref_link" src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==">
+  <img id="nested_image_label3" alt="image" aria-labelledby="crossref_link" src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==">
 </h3>
 
 <!-- self-referencial edge case-->