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/48910Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>