Update the link to the slides in README to correct url.
Update the link to the slides in README to correct url.

girldevelopit/gdi-featured-html-css-intro,Roenok/gdi-core-html-css,Roenok/gdi-core-html-css,girldevelopit/gdi-featured-html-css-intro,BergFulton/gdi-featured-html-css-intro,BergFulton/gdi-featured-html-css-intro