Resolve sylabs/singularity-admindocs#5

Restored brew instructions from sylabs/singularity-admindocs@86819f9

installation.rst

@@ -702,27 +702,19 @@ Install the following programs:
 Mac
 ---
 
-To use Singularity Desktop for macOS (Beta Preview):
-
-Download a Mac installer package `here
-<https://www.sylabs.io/singularity-desktop-macos/>`__.
-
-Singularity is also available via Vagrant (installable with
-`Homebrew <https://brew.sh>`_ or manually) or with the Singularity Desktop for
-macOS (Alpha Preview).
+Singularity is available via Vagrant (installable with
+`Homebrew <https://brew.sh>`_ or manually)
 
 To use Vagrant via Homebrew:
 
 .. code-block:: none
 
     $ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
-    $ brew install --cask virtualbox && \
-        brew install --cask vagrant && \
-        brew install --cask vagrant-manager
+    $ brew install --cask virtualbox vagrant vagrant-manager
 
------------------------        
-Singularity Vagrant Box
------------------------
+-------------------------        
+{Singularity} Vagrant Box
+-------------------------
 
 Run Git Bash (Windows) or open a terminal (Mac) and create and enter a
 directory to be used with your Vagrant VM.