Spellcheck settings: remove separator above first section

Remove vertical separator between subpage header and first section on the
Custom Spelling subpage of chrome://md-settings/languages.

R=stevenjb@chromium.org
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2557193002
Cr-Commit-Position: refs/heads/master@{#437701}

chrome/browser/resources/settings/languages_page/edit_dictionary_page.html

@@ -35,7 +35,7 @@
         -webkit-margin-start: 16px;
       }
     </style>
-    <div class="settings-box">
+    <div class="settings-box first">
       <iron-a11y-keys id="keys" keys="enter esc"
           on-keys-pressed="onKeysPress_"></iron-a11y-keys>
       <div class="start">