bug#27639: 25.2; Fix syntax for minor mode enabling in dir locals in manual

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Kaushal Modi <kaushal.modi@gmail.com> writes:

> I recently stumbled across this Reddit post[1] where a user is trying to
> enable a minor mode in .dir-locals.el. But he was doing that by setting the
> mode name to `t'.

The reddit thread talks about "custom-mode" which seems to be a major
mode.  Unless "custom-mode" was some kind of meta-variable for "a custom
mode that I created"?  It's a bit unclear.