readme: Update download link to 1.10.15

[skip ci]

README.md

@@ -24,7 +24,7 @@ Installing Panda3D
 ==================
 
 The latest Panda3D SDK can be downloaded from
-[this page](https://www.panda3d.org/download/sdk-1-10-14/).
+[this page](https://www.panda3d.org/download/sdk-1-10-15/).
 If you are familiar with installing Python packages, you can use
 the following command: