Fixing english typo in timebomb blurb. r=akkana

xpfe/components/timebomb/resources/locale/en-US/timebomb.dtd

@@ -1,4 +1,4 @@
 <!ENTITY     title.label             "Time Bomb Version">
 <!ENTITY     okButton.label          "OK">
 <!ENTITY     warnText.label          "This is a warning that this product will expire.  Please visit http://www.mozilla.org for newer versions.">
-<!ENTITY     expireText.label        "This is a version has expired.  You must download a newer version at http://www.mozilla.org.">
+<!ENTITY     expireText.label        "This version has expired.  You must download a newer version at http://www.mozilla.org.">