Fix typo in mix test task (#10767)
Fix typo in mix test task (#10767)

The --export-coverage option specify the result files. This commit make
it clear that it means the name of the file you want to export the
result file to.
kimshrier/elixir,michalmuskala/elixir,ggcampinho/elixir,kelvinst/elixir,joshprice/elixir,kimshrier/elixir,elixir-lang/elixir,ggcampinho/elixir,pedrosnk/elixir,kelvinst/elixir,pedrosnk/elixir