Fix typo in comment.

content/base/src/nsDocumentEncoder.cpp

@@ -1385,7 +1385,7 @@ nsHTMLCopyEncoder::PromoteAncestorChain(nsCOMPtr<nsIDOMNode> *ioNode,
     else
     {
       // passing parent as last param to GetPromotedPoint() allows it to promote only one level
-      // up the heirarchy.
+      // up the hierarchy.
       rv = GetPromotedPoint( kStart, *ioNode, *ioStartOffset, address_of(frontNode), &frontOffset, parent);
       NS_ENSURE_SUCCESS(rv, rv);
       // then we make the same attempt with the endpoint