README: Remove `track_files` from collate example

[runephilosof-abtion]

Remove

simplecov/README.md

Line 623 in b6c2d42

track_files "**/*.rb"

Or explain why it should be there (the PR introducing it does not mention why).

The example is using the rails profile which has

simplecov/lib/simplecov/profiles/rails.rb

Line 17 in b6c2d42

track_files "{app,lib}/**/*.rb"