doc: don't block page load on JavaScript fetch

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/12050045
diff --git a/doc/root.html b/doc/root.html
index e2d5341..e952a76 100644
--- a/doc/root.html
+++ b/doc/root.html
@@ -116,7 +116,7 @@
 	}
 }
 
-$(function() {
+window.initFuncs.push(function() {
 	// Set up playground.
 	playground({
 		"codeEl":        "#learn .code",