[x/blog] generate: update to new stringer format
[x/blog] generate: update to new stringer format

During code review for this post, dsymonds noticed a way to simplify
the generated code. That simplification is now checked in, so bring the
post up to date.

Change-Id: If29587332c8cc210ecac4843e9dd45dfa01118f7
Reviewed-on: https://go-review.googlesource.com/2020
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
X-Blog-Commit: b35b5e35dbb795b8392180ed8b008a2e245e55cf

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