a minor correction to read_pgm.m

[Wonkoo Kim]

I had to change fopen() line in read_pgm.m (posted a while ago) to

  [fid, msg] = fopen (filename, "rb");

as image files should be opened in binary mode under OS/2 (and
DOS/Win, ...).

(BTW, the octave manual doesn't explain binary mode "b" at all
 in fopen() description.  Ignoring non-unix users?  ;-) )

Thanks.

//--------------------------------------------------------------------
// Wonkoo Kim (address@hidden)