Fix a typo in pg_receivewal.c's get_destination_dir()

Hi,

It looks like there's a typo in pg_receivewal.c's
get_destination_dir(), that is, use the function parameter dest_folder
instead of global variable basedir in the error message. Although
there's no harm in it as basedir is a global variable in
pg_receivewal.c, let's use the function parameter to be in sync with
close_destination_dir.

Attaching a tiny patch to fix it.

Regards,
Bharath Rupireddy.