Bug 1268008 - Correct typo "certifiate" in certs_help.xhtml. r=IanN

suite/locales/en-US/chrome/common/help/certs_help.xhtml

@@ -237,7 +237,7 @@
 
 <p>To ensure that an entire
   <a href="glossary.xhtml#certificate_chain">certificate chain</a> of CAs are
-  all trusted, you need to edit the root CA certifiate only.</p>
+  all trusted, you need to edit the root CA certificate only.</p>
 
 <p>To import the chain, you click a link on a web page provided by the CA. You
   can then use the authorities tab to locate the root certificate and edit its