content/docker: update Dockerfile edit instructions

The outyet package already contains a `Dockerfile`.

Change-Id: I85ac3db2d80966db02d4d1ce9d2bcf2721f06016
Reviewed-on: https://go-review.googlesource.com/7460
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/content/docker.article b/content/docker.article
index 6d5727c..942bcf1 100644
--- a/content/docker.article
+++ b/content/docker.article
@@ -32,7 +32,7 @@
 
 * Write a Dockerfile
 
-Create a file named `Dockerfile` in the `outyet` directory with the following contents:
+Replace a file named `Dockerfile` in the `outyet` directory with the following contents:
 
 	# Start from a Debian image with the latest version of Go installed
 	# and a workspace (GOPATH) configured at /go.
@@ -71,7 +71,7 @@
 
 * Build and run the image
 
-Invoke Docker from the package directory to build an image using the `Dockerfile`:
+Invoke Docker from the `outyet` package directory to build an image using the `Dockerfile`:
 
 	$ docker build -t outyet .