update benchmark documentation to match code

[goatshriek]

The documentation for benchmarking includes a walkthrough of running performance tests on a piece of code, refactoring it, and testing for improvement.

While the content of the walkthrough is still relevant, the code snippet included for the performance test itself has not been updated to match the current performance test template, which is simpler to read and understand.

This is a simple change: update the benchmarking documentation code snippet for the CopyElement test to match the implementation in test/performance/element.cpp.

If you are up for it, refactor the test to make it even simpler by using the fixture function create_entry to construct an entry with elements, rather than doing this manually. This is optional though, and if you would rather focus on the mechanics of making changes to files and issuing a pull request, you are free to leave this change out.