Fix Elixir 1.17 warnings on single quotes

[fhunleth]

This fixes these new warnings:

$ mix compile
Compiling 1 file (.ex)
     warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
     │
 351 │     header = :io_lib.format('~n~2.2.0w:~2.2.0w:~9.6.0f ~p', [hour, min, sec, pid])
     │                             ~
     │
     └─ lib/extrace.ex:351:29

     warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
     │
 353 │     '#{header} #{body}\n'
     │     ~
     │
     └─ lib/extrace.ex:353:5