maint.mk: heads-up: ftpmirror urls are not quick to be updated

[Jim Meyering]

I uploaded coreutils several hours ago, and the http://ftpmirror.gnu.org...
URL that went into my announcement template still is not yet valid.
At least not for the mirror(s) I've been getting.
I manually changed it to the canonical URL.

Note that this works fine:
  http://ftp.gnu.org/gnu/coreutils/coreutils-8.2.tar.gz.sig
But this does not:
  http://ftpmirror.gnu.org/coreutils/coreutils-8.2.tar.gz.sig

I made a note of one slow-to-update mirror:
  ftp://sunsite.cnlab-switch.ch/mirror/gnu/coreutils/coreutils-8.2.tar.gz.sig