docs/for-each-ref: update pointer to color syntax

The documentation for the %(color) placeholder refers to the
color.branch.* config for more details. But those details
moved to their own section in b92c1a28f
(Documentation/config.txt: describe 'color' value type in
the "Values" section, 2015-03-03).  Let's update our
pointer. We can steal the text from 30cfe72d3 (pretty: fix
document link for color specification, 2016-10-11), which
fixed the same problem in a different place.

While we're at it, let's give an example, which makes the
syntax much more clear than just the text.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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