Grammar fix in FlatIndexHNSW (facebookresearch#4253)

Summary:
Changed "with with" to "with"

Pull Request resolved: facebookresearch#4253

Reviewed By: gtwang01

Differential Revision: D71857100

Pulled By: junjieqi

fbshipit-source-id: 6c11e3767cb0d244707c889206de10169fccd6bf

Author: m-bhatia

faiss/IndexHNSW.h

@@ -138,7 +138,7 @@ struct IndexHNSWPQ : IndexHNSW {
     void train(idx_t n, const float* x) override;
 };
 
-/** SQ index topped with with a HNSW structure to access elements
+/** SQ index topped with a HNSW structure to access elements
  *  more efficiently.
  */
 struct IndexHNSWSQ : IndexHNSW {