Fix typo.

Screenshot URL broken by previous commit; Fixing.

Author: Maikuolan

README.md

@@ -49,7 +49,7 @@ unset($Web, $FrontEnd, $Scanner, $Loader);
 ```
 
 __*Screenshot:*__
-![Screenshot](https://raw.githubusercontent.com/phpMussel/extras/master/screenshots/frontend-v3.0-alpha2.png)
+![Screenshot](https://raw.githubusercontent.com/phpMussel/extras/master/screenshots/frontend-v3.0.0-alpha2.png)
 
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