changed first "vocal" to "letters"

Author: orcutt989

steps/01-02-letters.html

@@ -4,7 +4,7 @@
 lang: en
 ---
 <p>
-  Now, try to find the vocal which appears in all words marked white but not in those marked red.
+  Now, try to find the letter which appears in all words marked white but not in those marked red.
   Enter the letter below to test your choice.
 </p>
 <div class="playfield verbose">