Fix obvious typo in string formatting

tools/gbench/util.py

diff --git a/tools/gbench/util.py b/tools/gbench/util.py
index 67b1e4c..bfce376 100644
--- a/tools/gbench/util.py
+++ b/tools/gbench/util.py
@@ -129,7 +129,7 @@
         thandle, output_name = tempfile.mkstemp()
         os.close(thandle)
         benchmark_flags = list(benchmark_flags) + \
-                          ['--benchmark_out=' % output_name]
+                          ['--benchmark_out=%s' % output_name]
 
     cmd = [exe_name] + benchmark_flags
     print("RUNNING: %s" % ' '.join(cmd))