doc/code.html: fix typos

Fixed example program's output to match program's code.
Fixes #5204.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/8365043

doc/code.html

@@ -234,7 +234,7 @@ You can now run the program by typing its full path at the command line:
 
 <pre>
 $ <b>$GOPATH/bin/hello</b>
-Hello!
+Hello, world.
 </pre>
 
 <p>
@@ -244,7 +244,7 @@ just type the binary name:
 
 <pre>
 $ <b>hello</b>
-Hello!
+Hello, world.
 </pre>
 
 <p>