Bug 330837 Fix sudo_in_progress to not say "an sudo"

patch by gijskruitbosch+bugs@gmail.com r=lpsolit a=myk

webtools/bugzilla/template/en/default/global/user-error.html.tmpl

@@ -1224,7 +1224,7 @@
 
   [% ELSIF error == "sudo_in_progress" %]
     [% title = "Session In Progress" %]
-    An sudo session (impersonating [% target FILTER html %]) is in progress.  
+    A sudo session (impersonating [% target FILTER html %]) is in progress.  
     End that session (using the link in the footer) before starting a new one.
 
   [% ELSIF error == "sudo_password_required" %]