fix(docs): use --cask in brew commands (NativeScript#1946)

* Remove cask from brew commands

It is no longer required to install cask packages with `brew cask`, since it has been integrated within brew itself in version 2.5.

See the release notes for more information
https://brew.sh/2020/09/08/homebrew-2.5.0/

* Add cask flag to brew commands

Author: Auke Geerts

docs/start/ns-setup-os-x.md

@@ -102,7 +102,7 @@ Complete the following steps to setup NativeScript on your macOS development mac
     <pre class="add-copy-button"><code class="language-terminal">brew tap AdoptOpenJDK/openjdk
     </code></pre>
 
-    <pre class="add-copy-button"><code class="language-terminal">brew cask install adoptopenjdk8
+    <pre class="add-copy-button"><code class="language-terminal">brew install --cask adoptopenjdk8
     </code></pre>
     
     2. Set the JAVA_HOME system environment variable.
@@ -113,7 +113,7 @@ Complete the following steps to setup NativeScript on your macOS development mac
     3. Install the [Android SDK](http://developer.android.com/sdk/index.html).
         1. In the terminal, run the following command:
 
-            <pre class="add-copy-button"><code class="language-terminal">brew cask install android-sdk
+            <pre class="add-copy-button"><code class="language-terminal">brew install --cask android-sdk
             </code></pre>
 
         2. Next, run the following command to set the ANDROID_HOME system environment variable: