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 <bradfitz@golang.org>

content/moretypes/slices-of-slice.go

diff --git a/content/moretypes/slices-of-slice.go b/content/moretypes/slices-of-slice.go
index 66f1019..8f5cbc8 100644
--- a/content/moretypes/slices-of-slice.go
+++ b/content/moretypes/slices-of-slice.go
@@ -18,7 +18,7 @@
 	// The players take turns.
 	board[0][0] = "X"
 	board[2][2] = "O"
-	board[2][0] = "X"
+	board[1][2] = "X"
 	board[1][0] = "O"
 	board[0][2] = "X"