fix typo in core/indexes.txt

Author: jrassi

source/core/indexes.txt

@@ -565,7 +565,7 @@ construction:
   larger than the available RAM, then the incremental process
   can take *much* longer than the foreground build.
 
-- If you your application includes :method:`ensureIndex()
+- If your application includes :method:`ensureIndex()
   <db.collection.ensureIndex()>` operations, and an index *doesn't*
   exist for other operational concerns, building the index can have a
   severe impact on the performance of the database.