gud-pdb-command-name wrong

[Dave Love]

The value of gud-pdb-command-name was changed from "pdb", which I
think was a result of my misunderstanding.  It should be changed back
to "pdb" since the output of the `pydb' variant of the debugger isn't
compatible.

The doc string could note that incompatibility.  It could also say
that if you don't have a "pdb" executable on your path, you can
probably use the absolute file name of the pdb module -- "pdb" is
normally a link to it.