Add "let's" to the whitelist

As discussed by e-mail, this change implements the proposal of
including a "lets" -> "let's" entry to the whitelist.

Change-Id: Ia7b7737c34ebf85675af78501acd865ce399f38d

java/res/values-en/whitelist.xml

@@ -34,5 +34,9 @@
         <item>thisd</item>
         <item>this\'d</item>
 
+        <item>255</item>
+        <item>lets</item>
+        <item>let\'s</item>
+
     </string-array>
 </resources>