Patch to fix docs for auto_link

Hi I've contributed a patch which fixes an incorrect example in the
documentation for auto_link in ActionView::Helpers::TextHelper.

The example uses truncate in the old style of tuncate(text, length)
which no longer works, so I've patched it to use the new
tuncate(text, :length => n) style.

Thanks :slight_smile:

Applied thank you very much.

If you prefer, in the future you may fix something like this directly
via docrails. docrails has public write access. Just clone, fix, and
push. The patch will be reviewed time permitting, but by default it
goes. docrails and rails/master are cross-merged periodically.