Fix docs bug stating file_fdw requires absolute paths

It has always (since the first commit) worked with relative paths, so
use the same wording as other parts of the documentation.

Author: Bruce Momjian
Discussion: https://postgr.es/m/CABUevExx-hm=cit+A9LeKBH39srvk8Y2tEZeEAj5mP8YfzNKUg@mail.gmail.com

doc/src/sgml/file-fdw.sgml

@@ -28,7 +28,8 @@
 
    <listitem>
     <para>
-     Specifies the file to be read.  Must be an absolute path name.
+     Specifies the file to be read.  Relative paths are relative to the
+     data directory.
      Either <literal>filename</literal> or <literal>program</literal> must be
      specified, but not both.
     </para>