Merge pull request #9394 from twbs/valid-github-btn-html

_includes/social-buttons.html: escape ampersands in URLs so HTML validates

_includes/social-buttons.html

 <div class="bs-social">
   <ul class="bs-social-buttons">
     <li>
-      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=twbs&repo=bootstrap&type=watch&count=true" allowtransparency="true" frameborder="0" scrolling="0" width="100px" height="20px" title="Star on GitHub"></iframe>
+      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=twbs&amp;repo=bootstrap&amp;type=watch&amp;count=true" allowtransparency="true" frameborder="0" scrolling="0" width="100px" height="20px" title="Star on GitHub"></iframe>
     </li>
     <li>
-      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=twbs&repo=bootstrap&type=fork&count=true" allowtransparency="true" frameborder="0" scrolling="0" width="102px" height="20px" title="Fork on GitHub"></iframe>
+      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=twbs&amp;repo=bootstrap&amp;type=fork&amp;count=true" allowtransparency="true" frameborder="0" scrolling="0" width="102px" height="20px" title="Fork on GitHub"></iframe>
     </li>
     <li class="follow-btn">
       <a href="https://twitter.com/twbootstrap" class="twitter-follow-button" data-link-color="#0069D6" data-show-count="true">Follow @twbootstrap</a>