text/tabwriter: fix BenchmarkPyramid and BenchmarkRagged again

text/tabwriter: fix BenchmarkPyramid and BenchmarkRagged again

These were added in CL 106979. I got them wrong.
They were fixed in CL 111643. They were still wrong.
Hopefully this change will be the last fix.

With this fix, CL 106979 is allocation-neutral for BenchmarkRagged.
The performance results for BenchmarkPyramid reported in CL 111643 stand.

Change-Id: Id6a522e6602e5df31f504adf5a3bec9969c18649
Reviewed-on: https://go-review.googlesource.com/116015
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>