(fix) adds missing period to line 24

readme.md

@@ -21,7 +21,7 @@ For preview release you can use the following ones:
 
 ## Preview release
 
-**3.0-alpha** can be found [here](https://github.com/BabylonJS/Babylon.js/tree/master/dist/preview%20release)
+**3.0-alpha** can be found [here](https://github.com/BabylonJS/Babylon.js/tree/master/dist/preview%20release).
 If you want to contribute, please read our [contribution guidelines](https://github.com/BabylonJS/Babylon.js/blob/master/contributing.md) first.
 
 ## Documentation