Center Oh My Zsh logo

Also, fix a couple of typos / style issues.

README.markdown

-![Oh My Zsh](https://s3.amazonaws.com/ohmyzsh/oh-my-zsh-logo.png)
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+<p align="center">
+  <img src="https://s3.amazonaws.com/ohmyzsh/oh-my-zsh-logo.png" alt="Oh My Zsh">
+</p>
 
 Oh My Zsh is an open source, community-driven framework for managing your [zsh](http://www.zsh.org/) configuration. That sounds boring. Let's try this again.
 
 __Oh My Zsh is a way of life!__ Once installed, your terminal prompt will become the talk of the town _or your money back!_ Each time you interact with your command prompt, you'll be able to take advantage of the hundreds of bundled plugins and pretty themes. Strangers will come up to you in cafés and ask you, _"that is amazing. are you some sort of genius?"_ Finally, you'll begin to get the sort of attention that you always felt that you deserved. ...or maybe you'll just use the time that you saved to start flossing more often.
 
-To learn more, visit [ohmyz.sh](http://ohmyz.sh) and/or follow [ohmyzsh](https://twitter.com/ohmyzsh) on Twitter.
+To learn more, visit [ohmyz.sh](http://ohmyz.sh) and follow [@ohmyzsh](https://twitter.com/ohmyzsh) on Twitter.
 
 ## Getting Started
 
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 ### Prerequisites
 
 __Disclaimer:__ _Oh My Zsh works best on Mac OS X and Linux._
 
 * Unix-based operating system (Mac OS X or Linux)
-* [Zsh](http://www.zsh.org) should be installed (v4.3.9 or more recent). If not pre-installed (`zsh --version` to confirm), check the following instruction here: [Installing-ZSH](https://github.com/robbyrussell/oh-my-zsh/wiki/Installing-ZSH)
+* [Zsh](http://www.zsh.org) should be installed (v4.3.9 or more recent). If not pre-installed (`zsh --version` to confirm), check the following instruction here: [Installing ZSH](https://github.com/robbyrussell/oh-my-zsh/wiki/Installing-ZSH)
 * `curl` or `wget` should be installed
 * `git` should be installed