strings: clarify example of ContainsAny

I have seen code that literally copied the example like this:

    if strings.ContainsAny(s, "1 & 2 & 3") {

The developer apparently thought that this is the way to
specify multiple characters and I noticed this pattern
being used in the example. Let's update the example so
that it's clear how multiple Unicode code points should
be specified.

Change-Id: Id4d780555e521af62fb787a7950be1e60848cd95
Reviewed-on: https://go-review.googlesource.com/c/go/+/178737


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>