StringIO.start -> StringIO.open

The function to create a StringIO is `open/1`, not `start/1` in the current versions of Elixir as confirmed by the docs and me just running through the Getting Started guide in iex.

```iex
iex(15)> {:ok, pid} = StringIO.start("hello")
** (UndefinedFunctionError) undefined function: StringIO.start/1
    (elixir) StringIO.start("hello")
iex(15)> l StringIO
{:module, StringIO}
iex(16)> {:ok, pid} = StringIO.start("hello")
** (UndefinedFunctionError) undefined function: StringIO.start/1
    (elixir) StringIO.start("hello")
iex(16)> StringIO.
close/1       contents/1    open/2
iex(16)> {:ok, pid} = StringIO.open("hello")
{:ok, #PID<0.68.0>}
```