text/scanner: use correct token position in example

text/scanner: use correct token position in example

While at it, unindent source text so column values are easier
to read, remove unnecessary text in output, and simplify the
loop.

Fixes #20346.

Change-Id: I0fde02b9e4242383da427f4cf4c6c13dd0ab3b47
Reviewed-on: https://go-review.googlesource.com/43450
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>