benchmark: harmonize progress bar + stderr output

Add a space for minimal readability.

PR-URL: https://github.com/nodejs/node/pull/11925
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/_benchmark_progress.js

@@ -104,7 +104,7 @@ class BenchmarkProgress {
           `| ${fraction(completedFiles, scheduledFiles)} files ` +
           `| ${fraction(completedRunsForFile, runsPerFile)} runs ` +
           `| ${fraction(completedConfig, scheduledConfig)} configs]` +
-          `: ${caption}`;
+          `: ${caption} `;
   }
 
   updateProgress(finished) {