Update 03-begin.html.md

The tutorial incorrectly suggests renaming the file `/files/styles/style.css.styl`, while the file was added in /documents folder in a previous step.
Merge pull request #91 from dsimunic/patch-1

The tutorial incorrectly suggests renaming the file
`/files/styles/style.css.styl`, while the file was added in /documents
folder in a previous step.