Fix typo

Signed-off-by: JonyFang <jony.chunfang@gmail.com>

Author: JonyFang

docs/CONTRIBUTING.md

@@ -8,7 +8,7 @@ We welcome any types of contribution, not only code. You can help by
 - **Q&A**:
 	- File issues that you're facing. The more detailed, the better. Offering the steps that can reproduce the issue is the best.
 - **Marketing**: 
-	- Writing blogs or posts, howto's tutorial and so on
+	- Writing blogs or posts, howto's tutorial and so on.
 - **Code**: 
 	- Firstly take a look at the [open issues](https://github.com/JonyFang/FFPopup/issues) or [TODO list](https://github.com/JonyFang/FFPopup#todo-list). You can change the code and raise a PR to us. Even if you couldn't write code, commenting on them, showing that you care about a given issue matters. It really helps.