posixmodule.c: fix chdir docstring

Fixes the following issue:

```
>>> from docutils.core import publish_doctree
>>> from posix import chdir
>>> publish_doctree(chdir.__doc__)
<string>:5: (ERROR/3) Unexpected indentation.
```

Author: haampie

Modules/posixmodule.c

@@ -3477,7 +3477,7 @@ Change the current working directory to the specified path.
 
 path may always be specified as a string.
 On some platforms, path may also be specified as an open file descriptor.
-  If this functionality is unavailable, using it raises an exception.
+If this functionality is unavailable, using it raises an exception.
 [clinic start generated code]*/
 
 static PyObject *