Update README.md

README.md

 # GitBook Default Theme
 
-This is the default theme for GitBook since version `3.0.0`. It can be used as a template for theming books or cam be extend.
+This is the default theme for GitBook since version `3.0.0`. It can be used as a template for theming books or can be extended.
 
 ![Image](https://raw.github.com/GitbookIO/theme-default/master/preview.png)