fix bug in code sample for Stream.resource

In the documentation for Stream.resource there's a sample code that
makes a reference to "File.close!", which doesn't exist.

Also, the pattern matching in the function passed as second argument
doesn't match {:ok, file}, so it should use "File.open!" instead.

Errors:
** (UndefinedFunctionError) undefined function: File.close!/1

** (FunctionClauseError) no function clause matching in :io.request/2
    io.erl:553: :io.request({:ok, #PID<0.50.0>}, {:get_line, :unicode,
[]})