io: fix typo inside copyBuffer
The "rt" seems to be caused after copy-pasting the previous "wt" block
which make sense as WriterTo, but for ReaderFrom it makes more sense
thinking of rf instead of rt.
Change-Id: I873699c27211bea6cdba3e199f36eb3c38188d70
GitHub-Last-Rev:
1795600a9b29946d824ba645c137da216bdf6302
GitHub-Pull-Request: golang/go#66811
Reviewed-on: https://go-review.googlesource.com/c/go/+/578635
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>