doc/articles/go_command: remove an extra word.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7061051

doc/articles/go_command.html

@@ -50,7 +50,7 @@ most other languages. Over the last few years we consistently reminded people
 about the <code>goinstall</code> command
 (now replaced by <a href="/cmd/go/#Download_and_install_packages_and_dependencies"><code>go get</code></a>)
 and its conventions: first, that the import path is derived in a known way from
-the URL of the source code; second, that that the place to store the sources in
+the URL of the source code; second, that the place to store the sources in
 the local file system is derived in a known way from the import path; third,
 that each directory in a source tree corresponds to a single package; and
 fourth, that the package is built using only information in the source code.