The old example was working on my Mac but when run on Linux I was finding that I had missing coverage information.
On some systems the old example of using xargs to pass list of profraw files can exceed xargs's argument limit resulting in multiple executions of llvm-profdata merge command which overwrites it's output file losing coverage information. This PR switches the example to using a file with a list of filenames.
There are other ways to solve this like using bash's globbing or a one liner with temporary file but I felt like this was the most simple, robust and easy to understand.
