fix formatting of folly benchmarks when --bm_relative_to flag is given

Summary: There was a missing newline separator

Reviewed By: yfeldblum

Differential Revision: D28781619

fbshipit-source-id: d20b26d67e427cf8760d21118e53a44ec6109771

folly/Benchmark.cpp

@@ -426,7 +426,7 @@ void printResultComparison(
   // Print header for a file
   auto header = [&](const string& file) {
     separator('=');
-    printf("%-*srelative  time/iter  iters/s", columns - 28, file.c_str());
+    printf("%-*srelative  time/iter  iters/s\n", columns - 28, file.c_str());
     separator('=');
   };