[x/tour] content: fix typo in strings.Reader slide
[x/tour] content: fix typo in strings.Reader slide

Fixes golang/go#13962

Change-Id: I2a35d67677bd4a4267fedf542428267ece22b52e
Reviewed-on: https://go-review.googlesource.com/18793
Reviewed-by: Francesc Campoy Flores <3149fa091f5edfa916be664d7a800f2d780f07b1@golang.org>
X-Tour-Commit: d117a43c4902b2e5ea407a63c8413b8befd81459

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website