Fix Guides search bar height

In latest Twitter Bootstrap versions form-control height is
no more auto and this change is needed to make our search
bar work properly, see: twbs/bootstrap#26820

guides/source/partials/_header.erb

@@ -13,11 +13,11 @@
     <form class="search-box" action="<%= url_for('/search/index.html') %>" novalidate>
       <div class="form-group form-group-submittable">
         <div class="has-float-label">
-          <input type="email" class="form-control placeholder-shown" name="inputSearch" id="inputSearch" placeholder="Search Solidus Documentation">
+          <input type="email" class="form-control h-auto placeholder-shown" name="inputSearch" id="inputSearch" placeholder="Search Solidus Documentation">
           <label for="inputSearch">Search Solidus Documentation</label>
           <button type="submit" class="btn btn-link"><%= inline_svg("icons/search.svg", class: "isvg") %></button>
         </div>
       </div>
     </form>
   </div>
-</header>
+</header>