update BrowserID to Persona in the prove_template email signature - not final...

update BrowserID to Persona in the prove_template email signature - not final but now it is less confusing and matches the url and from address

lib/browserid/prove_template.ejs

@@ -6,5 +6,5 @@
 <%= gettext('If you are NOT trying to sign into this site, just ignore this email.') %>
 
 <%= gettext('Thanks,') %>
-<%= gettext('BrowserID') %>
+<%= gettext('Persona') %>
 <%= gettext('(A better way to sign in)') %>