lib/godoc/example.html: htmlify the play code

Fixes #4539.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6942043

lib/godoc/example.html

@@ -8,7 +8,7 @@
 		{{$output := .Output}}
 		{{with .Play}}
 			<div class="play">
-				<div class="input"><textarea class="code">{{.}}</textarea></div>
+				<div class="input"><textarea class="code">{{html .}}</textarea></div>
 				<div class="output"><pre>{{html $output}}</pre></div>
 				<div class="buttons">
 					<a class="run" title="Run this code [shift-enter]">Run</a>