build-aux/announce-gen: amending gpg --verify command line

Hello,

When verifying detached signatures, the path to the signed file
should be supplied in the command line; in fact, gpg emits
a warning when it's absent, like

  gpg: assuming signed data in 'file.tar.gz'

Using the shorter form is a bad habit, it allows for an unintended
behavior when the .sig file contains a non-detached signature.

diff --git a/build-aux/announce-gen b/build-aux/announce-gen
index 356c034b2..829496a8a 100755
--- a/build-aux/announce-gen
+++ b/build-aux/announce-gen
@@ -652,7 +652,7 @@ Use a .sig file to verify that the corresponding file (without the
 .sig suffix) is intact.  First, be sure to download both the .sig file
 and the corresponding tarball.  Then, run a command like this:

-  gpg --verify $tarballs[0].sig
+  gpg --verify $tarballs[0].sig $tarballs[0]

 EOF
   my $gpg_fingerprint = `LC_ALL=C gpg --fingerprint $gpg_key_id | grep -v ^sub`;
@@ -695,7 +695,7 @@ As a last resort to find the key, you can try the official GNU
 keyring:

   wget -q https://ftp.gnu.org/gnu/gnu-keyring.gpg
-  gpg --keyring gnu-keyring.gpg --verify $tarballs[0].sig
+  gpg --keyring gnu-keyring.gpg --verify $tarballs[0].sig $tarballs[0]
 EOF

   print <<EOF;

-----BEGIN PGP SIGNATURE-----

iQFPBAEBCAA5FiEEvZ1N7nsv8cvvLuDE4KzT4Mvnh0oFAmqJxfUbFIAAAAAABAAO
bWFudTIsMi41KzEuMTIsMiwyAAoJEOCs0+DL54dKRiQIAKTSmhlS0vr2jYXdJrD/
ZEKFMk+OolUMe3Ly7eiGMDMdaqFhrZ1qDb8QRYm/AxSqloINjLvVRNz/8N0XNaAs
RoOciIbR9oKSNNiLKmwl8bUbXVhdHnhZrYzpHlZbGnYW/UCLcynBh2pbzWAaWDKf
XYIP+MpjEtyM1Rrg/ih45JVmkryOfIlYL6MtB98b+d29bJQGkOhRQqpsKYoBhkJ0
dCJcUA0Z5Xd3rqiC5Q7iWRBu/tIkU6zDswCMsyDWbbd3xyT3HS4IvEf30UZ46oq7
L9RJMekI9+nel7v1wNYBjbdf2A4fKoaC5bLvzBqRu7qwpsAtNs5Jp/WqbpapsTsn
aXQ=
=iXHg
-----END PGP SIGNATURE-----