do not allow less errors than upstream when building docs

Currently, 3 examples are failing on i386 with error message

  in `draw': pixels are not authentic `' @ error/cache.c/QueueAuthenticPixelCacheNexus/4387 (Magick::ImageMagickError)

When fail-on-doc-failure is applied, restricting the allowed errors from
3 to 1, the build fails on this architecture.
The patch is deactivated for the moment, until a more proper solution is
found.