contourc example

[Geordie McBain]

The example `c = contourc (x, y, z, linspace (0, 2*pi, 10));' in the
help string for contourc can't be run as it stands since x, y, and z
aren't defined.  This limits its usefulness somewhat.

I suggest a self-contained example, like

%<---
octave-2.9.10:1> contourc (x = 0:2, y = x, z = x' * y, 2:3) 
ans =

   2.0000   2.0000   1.0000   3.0000   1.5000   2.0000
   2.0000   1.0000   2.0000   2.0000   2.0000   1.5000
%<---

as in the attached patch.  The example also shows the (reasonably
concise) output.

contourc.diff
Description: Text Data