message: Fixes link to example html report
diff: If [gnuplot](http://www.gnuplot.info/) is installed, Criterion.rs can generate an HTML report displaying the results of the benchmark under `target/criterion/$BENCHMARK_NAME/`.
  -To see an example report, [click here](user_guide/html_report/index.html). For more details on the charts and statistics displayed, check the other pages of this book.
  \ No newline at end of file
  +To see an example report, [click here](html_report/index.html). For more details on the charts and statistics displayed, check the other pages of this book.
  