PSR-4 Autoloader class example contains typo

$file was immediately overwritten. The first entry did *nothing*.

The old code example:
```
$file = $base_dir
      . str_replace('\\', DIRECTORY_SEPARATOR, $relative_class)
      . '.php';
$file = $base_dir
      . str_replace('\\', '/', $relative_class)
      . '.php';
```

$file was never used and the first line had no side effects and thus did **nothing**.

Judging from the line from earlier in the class:

```
$base_dir = rtrim($base_dir, '/') . DIRECTORY_SEPARATOR;
$base_dir = rtrim($base_dir, DIRECTORY_SEPARATOR) . '/';
```

I have made the following change whereas the second `str_replace` is performed on $file rather than $relative_class