encoding/json: fix indentation in Decode stream example

encoding/json: fix indentation in Decode stream example

The existing example for Decoder.Decode (Stream) had excessive
indentation in the godoc interface for the const jsonStream,
making it hard to read. This fixes the indentation in the
example_test.go to improve the readability in godoc.

Helps #21026.

Change-Id: I16f56b82182da1dcc73cca44e535a7f5695e975d
Reviewed-on: https://go-review.googlesource.com/48910
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>