message: fix link to heat diffusion example
  Looks like relative links in github's readme don't work for .rs files.
diff: @@ -79,7 +79,7 @@ Matrix matrix multiplication, addition
  For a more comple example, be sure to check out the `heat diffusion`_ example.
  -_`heat diffusion`: examples/heat.rs
  +_`heat diffusion`: https://github.com/vbarrielle/sprs/blob/master/examples/heat.rs
  Documentation
  -------------
  