Fix shebang

[lemenkov]

Unfortunately a small typo slipped in PR #14343. it was unnoticed because if you run bin/elixir ./lib/elixir/scripts/cover.exs sheband won't be used at all.

[maennchen]

@lemenkov This is referring to bin/elixir inside the current project.

For the global elixir binary, I would've used /usr/bin/env elixir.

[josevalim]

Closing per explanation above. :)