doc: Proper default value for the $GOPATH.

It should be $HOME/go instead of $HOME/work

Change-Id: I22e0a89deff30f935c3e25a237d6c0c161103339
Reviewed-on: https://go-review.googlesource.com/46890Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/code.html

@@ -270,7 +270,7 @@ This command builds the <code>hello</code> command, producing an executable
 binary. It then installs that binary to the workspace's <code>bin</code>
 directory as <code>hello</code> (or, under Windows, <code>hello.exe</code>).
 In our example, that will be <code>$GOPATH/bin/hello</code>, which is
-<code>$HOME/work/bin/hello</code>.
+<code>$HOME/go/bin/hello</code>.
 </p>
 
 <p>