error-values.md: fix example output
error-values.md: fix example output

Modify the example for detail-mode output to match the current implementation.

Change-Id: I0109fd6f51e787da06772b589d17dd7f73793629
Reviewed-on: https://go-review.googlesource.com/c/160977
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/proposal,golang/proposal