Update CONTRIBUTING.md: Fix typo

CONTRIBUTING.md

@@ -45,7 +45,7 @@ With this command you can specify a file or folder to limit which files it will
 
 ### Keep documentation in sync
 
-Pico accepts the problems of having redundant documentation on different places (concretely Pico's inline user docs, the `README.md` and the website) for the sake of a better user experience. When updating the docs, please make sure the keep them in sync.
+Pico accepts the problems of having redundant documentation on different places (concretely Pico's inline user docs, the `README.md` and the website) for the sake of a better user experience. When updating the docs, please make sure to keep them in sync.
 
 If you update the [`README.md`](https://github.com/picocms/Pico/blob/master/README.md) or [`content-sample/index.md`](https://github.com/picocms/Pico/blob/master/content-sample/index.md), please make sure to update the corresponding files in the [`_docs`](https://github.com/picocms/Pico/tree/gh-pages/_docs/) folder of the `gh-pages` branch (i.e. [Pico's website](http://picocms.org/docs.html)) and vice versa. Unfortunately this involves three (!) different markdown parsers. If you're experiencing problems, use Pico's [`erusev/parsedown-extra`](https://github.com/erusev/parsedown-extra) as a reference. You can try to make the contents compatible to [Redcarpet](https://github.com/vmg/redcarpet) by yourself, otherwise please address the issues in your pull request message and we'll take care of it.