Update index.html

This is very minor, but to help encourage best practises it'd be best to have the input type 'email' for this email input example instead of 'text'

docs/examples/signin/index.html

@@ -32,7 +32,7 @@
 
       <form class="form-signin" role="form">
         <h2 class="form-signin-heading">Please sign in</h2>
-        <input type="text" class="form-control" placeholder="Email address" required autofocus>
+        <input type="email" class="form-control" placeholder="Email address" required autofocus>
         <input type="password" class="form-control" placeholder="Password" required>
         <label class="checkbox">
           <input type="checkbox" value="remember-me"> Remember me