Fixes bootstrap js ref to 4.3.1 for API v1.15

A fix for the v1.15 API version was missed in the kubernetes#14775 PR
because the dev-1.15 branch was forked from the master branch
before the fix and has been merged back to the master after it.

static/docs/reference/generated/kubernetes-api/v1.15/index.html

@@ -55697,7 +55697,7 @@ <H1 id="webhookclientconfig-v1alpha1-auditregistration-k8s-io">WebhookClientConf
 </DIV>
 <SCRIPT src="/js/jquery-3.2.1.min.js"></SCRIPT>
 <SCRIPT src="jquery.scrollTo.min.js"></SCRIPT>
-<SCRIPT src="/js/bootstrap-3.3.7.min.js"></SCRIPT>
+<SCRIPT src="/js/bootstrap-4.3.1.min.js"></SCRIPT>
 <SCRIPT src="navData.js"></SCRIPT>
 <SCRIPT src="scroll.js"></SCRIPT>
 </BODY>