Update README.md

Updated the SPO release v4.15

Author: bramdejager

README.md

@@ -14,7 +14,7 @@ When selecting the release, download the release which is corresponding your tar
 
 | Target system     | Client                            | Version | Latest Release                                                      |
 | ---               | ---                               | :---:   | :---:                                                               |
-| SharePoint Online | SharePoint Online Client Browser  | v4.x    | [download](https://github.com/bramdejager/spcb/releases/tag/v4.13)   |
+| SharePoint Online | SharePoint Online Client Browser  | v4.x    | [download](https://github.com/bramdejager/spcb/releases/tag/v4.15)   |
 | SharePoint 2019   | SharePoint 2019 Client Browser    | v3.x    | [download](https://github.com/bramdejager/spcb/releases/tag/v3.12)   |
 | SharePoint 2016   | SharePoint 2016 Client Browser    | v2.x    | [download](https://github.com/bramdejager/spcb/releases/tag/v2.13)  |
 | SharePoint 2013   | SharePoint 2013 Client Browser    | v1.x    | [download](https://github.com/bramdejager/spcb/releases/tag/v1.20)  |