Update understanding conformance partial 3rd party section id (#4657)

Makes the link in
https://www.w3.org/WAI/WCAG22/Understanding/conformance under
"Understanding Requirement 2" second note work.

Opted to change `id` rather than the link target, as this then maps
directly to the way https://www.w3.org/TR/WCAG22/#conformance-partial is
linked to in the normative spec

Closes #4654

Co-authored-by: Mike Gower <[email protected]>

Author: patrickhlauke

understanding/conformance.html

@@ -283,7 +283,7 @@ <h2>Understanding Conformance Claims</h2>
 
       </div>
 
-      <section id="partial-third">
+      <section id="conformance-partial">
 
          <h3>Partial conformance claims due to third party content</h3>