Update benchmarks

[3flex]

benchmarks.md states that to run a benchmark to compare a new implementation against cmark that the command make bench PROG=/path/to/program should be run.

It also says that the input file is an 11MB Markdown file, however when that command is run the input file is ~110MB since 79603ef changed things to concatenate Progit in all languages not once but ten times.

Can the statement be corrected and benchmarks updated? Thanks!