encoding/json: de-indent raw strings in remaining examples

encoding/json: de-indent raw strings in remaining examples

This change fixes the remaining examples where the raw strings had
suboptimal indentation (one level too many) when viewed in godoc.

Follows CL 48910.
Fixes #21026.

Change-Id: Ifc0dae3fa899a9fff8b1ff958414e2fe6852321d
Reviewed-on: https://go-review.googlesource.com/50990
Run-TryBot: Dmitri Shuralyov <shurcool@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>