Fix typo (#541)

Author: xMartin

src/api/server/authn.rst

@@ -274,7 +274,7 @@ Proxy Authentication
 
 .. note::
     To use this authentication method make sure that the
-    ``{chttpd_auth, proxy_authentication_handler}`` value in added to the
+    ``{chttpd_auth, proxy_authentication_handler}`` value is added to the
     list of the active :config:option:`chttpd/authentication_handlers`:
 
     .. code-block:: ini