content: remove unnecessary ampersand

This example showed an interface being satisfied by *T when
the method is declared on T.

Fixes golang/go#14344

Change-Id: Ifcfd251d57323714539a90e17fdb1e74531e3424
Reviewed-on: https://go-review.googlesource.com/19531
Reviewed-by: Katrina Owen <katrina.owen@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>