Update concurrency.md (#2259)
Update concurrency.md (#2259)

Fixed of minor details:
- just as #2246 and #2243 - add parentheses;
- fixed if variable is unsed, prefix it with an underscore;
- update with example Task (add owner with value);
nscyclone/elixir-school,doomspork/elixir_school,doomspork/elixir_school,doomspork/elixir-school,nscyclone/elixir-school,doomspork/elixir-school,doomspork/elixir-school,nscyclone/elixir-school,doomspork/elixir_school,nscyclone/elixir-school