Fix path to icons

https://discourse.appimage.org/t/correct-icons-path-in-an-appimage-tree/334/3?u=probono

README.md

@@ -55,6 +55,7 @@ You'll need to provide the basic structure of an `AppDir` which should look some
         └── icons
             └── <theme>
                 └── <resolution> 
+                    └── apps 
                         └── your_app.png
 ```
 Replace `<theme>` and `<resolution>` with (for example) `hicolor` and `256x256` respectively; see [icon theme spec](https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html) for more details.