content: fix typo in flowcontrol/2
content: fix typo in flowcontrol/2

Fixes golang/tour#575

Change-Id: Ic86fcddbd72b6a54e75b794b0b760151e2a4b78c
Reviewed-on: https://go-review.googlesource.com/128536
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

Go-zh/tour,golang/tour,dupoxy/go-tour-fr,dupoxy/go-tour-fr,Go-zh/tour,dupoxy/go-tour-fr,zorion/go-tour-ca,zorion/go-tour-ca,Go-zh/tour,Go-zh/tour