content: make slice of slice example (tic tac toe) less ambiguous
content: make slice of slice example (tic tac toe) less ambiguous

Fixes golang/go#14871.

Change-Id: Idbc9622e7acc216275620cc0036b0f116777ae71
Reviewed-on: https://go-review.googlesource.com/20886
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

dupoxy/go-tour-fr,golians/tour,scryner/tour,scryner/tour,scryner/tour,shuLhan/go-tour-id,dupoxy/go-tour-fr,zorion/go-tour-ca,shuLhan/tour,golians/tour,Go-zh/tour,Go-zh/tour,zorion/go-tour-ca,shuLhan/tour,shuLhan/go-tour-id,shuLhan/tour,scryner/tour,golang/tour,Go-zh/tour,shuLhan/go-tour-id,golians/tour,Go-zh/tour,dupoxy/go-tour-fr