Small adjustment on sign_out function (#603)

* Small adjustment on sing_out function

First, thanks for this. It has been a great help. I was having trouble with this function, getting a TypeError (TypeError: can only concatenate str (not "NoneType") to str) because apparently the second os.remove(session_cache_path()) is trying to delete a file that does not exist anymore so this is what worked for me.

* Change description on app.py file

* Update app.py on line 79

Fixed indent

CHANGELOG.md

@@ -8,7 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-// Add your changes here and then delete this line
+### Changed
+- moved os.remove(session_cache_path()) inside try block to avoid TypeError on app.py example file
 
 ## [2.16.1] - 2020-10-24