doc: improve the interface example in Effective Go

doc: improve the interface example in Effective Go

The String method is n-squared and overwrites its receiver.
Fix both issues, with only a slight loss of clarity.

Fixes #28773

Change-Id: I588f69d4cbd72931b28b984671512834473bd466
Reviewed-on: https://go-review.googlesource.com/c/151217
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>