chrome/settings: Fix the name of an i18n key I forgot to fully update.

R=khorimoto@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1166003003

Cr-Commit-Position: refs/heads/master@{#333103}

chrome/browser/resources/settings/a11y_page/a11y_page.html

@@ -59,7 +59,7 @@
       </div>
 
       <cr-settings-checkbox pref="{{prefs.settings.a11y.virtual_keyboard}}"
-          i18n-values="label:OnScreenKeyboardLabel">
+          i18n-values="label:onScreenKeyboardLabel">
       </cr-settings-checkbox>
 </if>