Remove sr-only from components/dropdowns example

Assuming sr-only is used in the actual live code so that the menu can be
shown open already ... but having sr-only in the highlighted example
code itself is confusing/misleading.

Also, change the text for the button from "Dropdown" to "Dropdown
trigger" for clarity (and it then matches
http://getbootstrap.com/javascript/#dropdowns)

docs/_includes/components/dropdowns.html

@@ -8,7 +8,7 @@
   <div class="bs-example">
     <div class="dropdown clearfix">
       <button class="btn dropdown-toggle sr-only" type="button" id="dropdownMenu1" data-toggle="dropdown">
-        Dropdown
+        Dropdown trigger
         <span class="caret"></span>
       </button>
       <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
@@ -22,8 +22,8 @@
   </div><!-- /example -->
 {% highlight html %}
 <div class="dropdown">
-  <button class="btn dropdown-toggle sr-only" type="button" id="dropdownMenu1" data-toggle="dropdown">
-    Dropdown
+  <button class="btn dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown">
+    Dropdown trigger
     <span class="caret"></span>
   </button>
   <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">