content: clarify newton formula and remove unused image

Add subscripts to the z variable to make the formula
more mathematical.

Also remove an old image (newton3.png) that was used for
an exercise that got deleted in golang.org/cl/111480043

Fixes golang/go#11306

Change-Id: I0b9f9f35a2fe329db4b59b7b16d1c1e98d6f9bf0
Reviewed-on: https://go-review.googlesource.com/15682
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed