docs: Make consistent use of optional li closing tags.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/105100044

doc/docs.html

@@ -147,8 +147,8 @@ Guided tours of Go programs.
 <li><a href="/blog/godoc-documenting-go-code">Godoc: documenting Go code</a> - writing good documentation for <a href="/cmd/godoc/">godoc</a>.</li>
 <li><a href="/blog/profiling-go-programs">Profiling Go Programs</a></li>
 <li><a href="/doc/articles/race_detector.html">Data Race Detector</a> - a manual for the data race detector.</li>
-<li><a href="/blog/race-detector">Introducing the Go Race Detector</a> - an introduction to the race detector.
-<li><a href="/doc/asm">A Quick Guide to Go's Assembler</a> - an introduction to the assembler used by Go.
+<li><a href="/blog/race-detector">Introducing the Go Race Detector</a> - an introduction to the race detector.</li>
+<li><a href="/doc/asm">A Quick Guide to Go's Assembler</a> - an introduction to the assembler used by Go.</li>
 </ul>
 
 <h4 id="articles_more">More</h4>