Fix comment of Benchmark.h

Summary:
Remove one of the "one" in the comment of BENCHMARK

Closes: #140

Test Plan: Inspection.

Reviewed By: markisaa@fb.com

Subscribers: trunkagent, folly-diffs@, yfeldblum

FB internal diff: D1877997

Signature: t1:1877997:1425063475:c53c09d105809de9a9070f464fb0571a4dd1e5d1

folly/Benchmark.h

@@ -284,11 +284,11 @@ void doNotOptimizeAway(T&& datum) {
   static unsigned funName(paramType paramName)
 
 /**
- * Introduces a benchmark function. Use with either one one or two
- * arguments. The first is the name of the benchmark. Use something
- * descriptive, such as insertVectorBegin. The second argument may be
- * missing, or could be a symbolic counter. The counter dictates how
- * many internal iteration the benchmark does. Example:
+ * Introduces a benchmark function. Use with either one or two arguments.
+ * The first is the name of the benchmark. Use something descriptive, such
+ * as insertVectorBegin. The second argument may be missing, or could be a
+ * symbolic counter. The counter dictates how many internal iteration the
+ * benchmark does. Example:
  *
  * BENCHMARK(vectorPushBack) {
  *   vector<int> v;