util/ribbon_alg.h: removed duplicate word "vector" (#9216)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/9216

Reviewed By: riversand963

Differential Revision: D36219934

fbshipit-source-id: 8253b4e3eacceb8b040eeaa45cd5a50570a4eba6

util/ribbon_alg.h

@@ -142,7 +142,7 @@ namespace ribbon {
 // only some small fixed number of columns (typically k=3) to 1 for each
 // row of C, with remaining entries implicitly 0. This is implemented as
 // three hash functions over [0,m), and S can be implemented as a vector
-// vector of b-bit values. Now, a query only involves looking up k rows
+// of b-bit values. Now, a query only involves looking up k rows
 // (values) in S and computing their bitwise XOR. Additionally, this
 // construction can use a linear time algorithm called "peeling" for
 // finding a solution in many cases of one existing, but peeling