Update to available version of JDK8

Homebrew/homebrew-cask-versions#7253

doc/install_macosx.md

@@ -5,7 +5,7 @@
 2. Install dependencies:
     1. `brew install python3`
     2. `brew install ffmpeg boost-python3`
-    3. `brew cask install java8`
+    3. `brew cask install homebrew/cask-versions/adoptopenjdk8`
 
 3. If you are installing from source (and not using "pip3 install malmo"), unzip the Malmo zip to some location (e.g. your home folder as assumed in the next step).
 4. Add `export MALMO_XSD_PATH=~/MalmoPlatform/Schemas` (or your Schemas location) to your `~/.bashrc` and do `source ~/.bashrc`