corrected minor typo in readme file

README.md

@@ -872,7 +872,7 @@ Reveal.initialize({
 
 ## PDF Export
 
-Presentations can be exported to PDF via a special print stylesheet. This feature requires that you use [Google Chrome](http://google.com/chrome) or [Chromium](https://www.chromium.org/Home) and to be serving the presention from a webserver.
+Presentations can be exported to PDF via a special print stylesheet. This feature requires that you use [Google Chrome](http://google.com/chrome) or [Chromium](https://www.chromium.org/Home) and to be serving the presentation from a webserver.
 Here's an example of an exported presentation that's been uploaded to SlideShare: http://www.slideshare.net/hakimel/revealjs-300.
 
 ### Page size