encoding/json: fix package shadowing in MarshalIndent example

Prior to this CL, pasting the example from the website causes a
compilation error for some programs because it was shadowing the
"json" package.

Change-Id: I39b68a66ca99468547f2027a7655cf1387b61e95
Reviewed-on: https://go-review.googlesource.com/c/go/+/301492


Reviewed-by: Joe Tsai <joetsai@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Joe Tsai <joetsai@google.com>
Run-TryBot: Joe Tsai <joetsai@google.com>
TryBot-Result: Go Bot <gobot@golang.org>