fix hnsw shrink_neighbor_list comment (facebookresearch#2980)

Summary:
This pr is to fix the issue facebookresearch#2978 .

Pull Request resolved: facebookresearch#2980

Reviewed By: mdouze

Differential Revision: D47950592

Pulled By: mlomeli1

fbshipit-source-id: 32ef06c3775f7234a5a4bb4dab36c176edea2d1f

faiss/impl/HNSW.cpp

@@ -222,7 +222,7 @@ int HNSW::prepare_level_tab(size_t n, bool preset_levels) {
     return max_level;
 }
 
-/** Enumerate vertices from farthest to nearest from query, keep a
+/** Enumerate vertices from nearest to farthest from query, keep a
  * neighbor only if there is no previous neighbor that is closer to
  * that vertex than the query.
  */