Corrected another minor typo in a code comment.

js/leafy_tree.js

@@ -11,7 +11,7 @@ export class Monoid {
   }
 }
 
-// Likewise, we not only give an ordered monoid a way to compare its summaries (combineSummaries), we also include a function (summarizePosition) to convert
+// Likewise, we not only give an ordered monoid a way to compare its summaries (compareSummaries), we also include a function (summarizePosition) to convert
 // from a position type to the summary type.  That way, leafy trees can have one type for positions and a different type for summaries.  (Yet again, if
 // different types are not required, then summarizePosition can just be set to the identity function.)
 //