Bug 33482: Update about:tor donate string

Replace "Keep Tor strong"
with    "Tor is free to use because of donations from people like you"

chrome/content/aboutTor/aboutTor.xhtml

@@ -66,7 +66,7 @@
     </div>
 
     <div id="bottom">
-      <p id="donate" class="moreInfoLink">&aboutTor.donationBanner.line2e;
+      <p id="donate" class="moreInfoLink">&aboutTor.donationBanner.freeToUse;
         <a href="https://donate.torproject.org/">&aboutTor.donationBanner.buttonA; &#187;</a>
       </p>
       <p id="manual" class="showForManual moreInfoLink">&aboutTor.torbrowser_user_manual_questions.label;

chrome/locale/en-US/aboutTor.dtd

@@ -27,5 +27,5 @@
 
 <!ENTITY aboutTor.newsletter.tagline "Get the latest news from Tor straight to your inbox.">
 <!ENTITY aboutTor.newsletter.link_text "Sign up for Tor News.">
-<!ENTITY aboutTor.donationBanner.line2e "Keep Tor strong.">
+<!ENTITY aboutTor.donationBanner.freeToUse "Tor is free to use because of donations from people like you.">
 <!ENTITY aboutTor.donationBanner.buttonA "Donate Now">