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>
diff --git a/content/img/newton.png b/content/img/newton.png
index 540ec83..3691e57 100644
--- a/content/img/newton.png
+++ b/content/img/newton.png
Binary files differ
diff --git a/content/img/newton3.png b/content/img/newton3.png
deleted file mode 100644
index 526a1e5..0000000
--- a/content/img/newton3.png
+++ /dev/null
Binary files differ