Fixes bootstrap js ref to 4.3.1 for API v1.15 (#16263)

A fix for the v1.15 API version was missed in the #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 @@ $ curl -X GET 'http://127.0.0.1:8001/apis/extensions/v1beta1/watch/namespaces/de
 </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>