fix: inconsistent file names in README.md

[franciscobmacedo]

Fixes #426

In the example calendar component this is what changed:

• css file name tostyle.css and the css variable in the Media class to calendar/style.css
• js file name to script.js and the js variable in the Media class to calendar/script.js
• the template_namevariable to calendar/template.html
• replaced the file structure screenshot with a file tree with the above files
• updated comments referencing this files

[EmilStenstrom]

Thanks @franciscobmacedo, really appreciate it!