Update duplicate heading IDs to match existing spec

Author: foolip

index.bs

@@ -1267,7 +1267,7 @@ when it receives a particular <a>command</a>.
 </section> <!-- /Errors -->
 
 <section>
-<h3 id=extensions>Extensions</h3>
+<h3 id=extensions-0>Extensions</h3>
 
 <p>Using the terminology defined in this section, others may define additional
  commands that seamlessly integrate with the standard protocol. This allows
@@ -6885,7 +6885,7 @@ The first argument provided to the function will be serialized to JSON and retur
 </aside> <!-- /example -->
 
 <section>
-<h3 id=input-sources>Input sources</h3>
+<h3 id=input-sources-0>Input sources</h3>
 
 <p class=note>
 The objects and properties defined in this section are spec-internal constructs