doc: fix some HTML errors in code.html

R=golang-dev, rsc
CC=adg, golang-dev
https://golang.org/cl/5720050

doc/code.html

@@ -53,7 +53,7 @@ command executables. It has a prescribed structure of three subdirectories:
 <ul>
 <li><code>src</code> contains Go source files,
 <li><code>pkg</code> contains compiled package objects, and
-<li><code>bin</code> contains executable commands.</code>
+<li><code>bin</code> contains executable commands.
 </ul>
 
 <p>
@@ -204,7 +204,7 @@ Go's naming conventions.
 </p>
 
 
-<h2>Building and installing</h3>
+<h2>Building and installing</h2>
 
 <p>
 The <code>go</code> command comprises several subcommands, the most central being
@@ -406,7 +406,7 @@ Run <code><a href="/cmd/go/#Test_packages">go help test</a></code> and see the
 </p>
 
 
-<h2 id="remote">Remote packages</h3>
+<h2 id="remote">Remote packages</h2>
 
 <p>
 An import path can describe how to obtain the package source code using a