This fixes a syntax error in plot-timer-events script.

Before:
  $ out/x64.release/d8 --prof --log-timer-events script.js
  $ tools/plot-timer-events v8.log
  tools/plot-timer-events: 75: tools/plot-timer-events: options+=1343: not found
  --distortion=
  Cmdline args: [options] [log-file-name]
  Default log file name is "v8.log".
    ...

After:
  $ out/x64.release/d8 --prof --log-timer-events script.js
  $ tools/plot-timer-events v8.log
  $ ls timer-events.png
  timer-events.png

BUG=

Review-Url: https://codereview.chromium.org/1961873002
Cr-Commit-Position: refs/heads/master@{#36434}