docs: Delete `brew update` from docs

Fix #3518

docs/content/overview/installing.md

@@ -37,7 +37,7 @@ Ideally, you should install it somewhere in your `PATH` for easy use.
 `/usr/local/bin` is the most probable location.
 
 On macOS, if you have [Homebrew](http://brew.sh/), installation is even
-easier: just run `brew update && brew install hugo`.
+easier: just run `brew install hugo`.
 
 For a more detailed explanation follow the corresponding installation guides:
 

docs/content/tutorials/installing-on-mac.md

@@ -60,26 +60,22 @@ When I did this, I had some problems with directory permissions. Searches on Goo
 
 ### Step 2: Run the `brew` command to install `hugo`
 
-First, update the formulae and Homebrew itself by running:
-
-```bash
-$ brew update
-```
-
-Then, install Hugo using Homebrew by running:
-
-
 ```bash
 $ brew install hugo
-==> Downloading https://homebrew.bintray.com/bottles/hugo-0.13_1.yosemite.bottle.tar.gz
-######################################################################## 100.0%
-==> Pouring hugo-0.13_1.yosemite.bottle.tar.gz
-🍺  /usr/local/Cellar/hugo/0.13_1: 4 files,  14M
+==> Downloading https://homebrew.bintray.com/bottles/hugo-0.21.sierra.bottle.tar.gz
+######################################################################## 100,0%
+==> Pouring hugo-0.21.sierra.bottle.tar.gz
+==> Using the sandbox
+==> Caveats
+Bash completion has been installed to:
+  /usr/local/etc/bash_completion.d
+==> Summary
+🍺  /usr/local/Cellar/hugo/0.21: 32 files, 17.4MB
 ```
 
 (Note: Replace `brew install hugo` with `brew install hugo --HEAD`
 if you want the absolute latest version in development,
-but beware—there might be bugs!)
+but beware — there might be bugs!)
 
 `Brew` should have updated your path to include Hugo. Confirm by opening a new terminal window and running a few commands:
 

docs/layouts/index.html

@@ -254,7 +254,7 @@
                 </a>
                 <p>&nbsp;</p>
                 <h4>Using Homebrew?</h4>
-                <pre><code>brew update &amp;&amp; brew install hugo</code></pre>
+                <pre><code>brew install hugo</code></pre>
               </div>
             </div>
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