yaml_to_cpp example code saves a file to current working directory

[kylebaron]

This was discovered on CRAN pretest. I didn't see it prior to submitting, but I didn't run check with --as-cran; you can see it then.

Found the following files/directories:
  ‘file60fa5b946f7b.yaml.mod’
* checking for detritus in the temp directory ... OK
* DONE

Status: 1 NOTE
See
  ‘/Users/kyleb/git/m4solve/mrgsolve.Rcheck/00check.log’
for details.

With the updated example code, you don't get the note.