Stop calling the cards 'slides' in some places.

flashcards.html

@@ -20,8 +20,8 @@
       </label>
       <div id=adv-settings>
         <label>
-          <span>Seconds per slide:</span>
-          <input type=number name=slidePeriod value=3>
+          <span>Seconds per card:</span>
+          <input type=number name=cardPeriod value=3>
         </label>
         <label>
           <span>Show cards </span>