Make main segmented button keyboard-focusable

unless there's a reason why ``tabindex="-1"`` was dropped in there...

docs/_includes/components/input-groups.html

@@ -261,7 +261,7 @@ <h2 id="input-groups-buttons-segmented">Segmented buttons</h2>
       <div class="col-lg-6">
         <div class="input-group">
           <div class="input-group-btn">
-            <button type="button" class="btn btn-default" tabindex="-1">Action</button>
+            <button type="button" class="btn btn-default">Action</button>
             <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-expanded="false">
               <span class="caret"></span>
               <span class="sr-only">Toggle Dropdown</span>
@@ -281,7 +281,7 @@ <h2 id="input-groups-buttons-segmented">Segmented buttons</h2>
         <div class="input-group">
           <input type="text" class="form-control" aria-label="Text input with segmented button dropdown">
           <div class="input-group-btn">
-            <button type="button" class="btn btn-default" tabindex="-1">Action</button>
+            <button type="button" class="btn btn-default">Action</button>
             <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-expanded="false">
               <span class="caret"></span>
               <span class="sr-only">Toggle Dropdown</span>