Fix Code Splitting example for ClojureScirpt 1.11+

[dawranliou]

This PR fixes the following two things:

Incorrect script paths in Index.html

Figwheel serves the static files from the target/public/ folder so the
target prefix in the target/cljs-out/*.js script src paths are incorrect.
This commit fixes the script paths in the index.html host page so the browser
doesn't show 404 responses for the compiled js files.

goog.dom.createDom error in cljs 1.11+

The goog.dom/createDom function requires a second argument of
opt_attributes. (See official documentation at
https://google.github.io/closure-library/api/goog.dom.html.) In previous
ClojureScript versions, the goog.dom/createDom function was able to handle the
nonexistence of opt_attributes argument, miraculously. However, with
ClojureScript 1.11.60, this is considered an error:

dom.js:171 Uncaught TypeError: Cannot set property assignedSlot of #<Element> which has only a getter
    at dom.js:171:20
    at Object.forEach (object.js:11:7)
    at goog.dom.setProperties (dom.js:156:15)
    at goog.dom.createDom_ (dom.js:264:16)
    at goog.dom.createDom (dom.js:252:19)
    at foo$core$listen_to_button (core.cljs:11:17)
    at core.cljs:23:16

This PR fixes the errors above and is tested on both ClojureScript 1.11.60
and ClojureScript 1.10.773.

See https://github.com/dawranliou/figwheel-main-code-splitting-demo for the
working demo.