allowed_signers: point the examples at 3.1.6

The comment block told the reader to run "git verify-tag 3.1.5", which is
exactly the tag that fails: 3.1.5 was tagged by the GitHub release form as
a lightweight ref and carries no signature. Use 3.1.6 in the examples and
state the baseline, so the file that teaches verification does not hand out
a command that cannot work.

https://github.com/acmesh-official/acme.sh/issues/7273

allowed_signers

@@ -4,17 +4,19 @@
 # maintainer and is never available to CI, so a compromise of the build
 # pipeline cannot produce a tag that verifies against this file.
 #
+# Signing starts at 3.1.6; tags up to 3.1.5 are unsigned.
+#
 # Fingerprint: SHA256:M60qVafm/NUywQHXAkoQcj2v6KgkfrdSXv6mPejUUeE
 #
 # To verify a release tag, from a clone of this repository:
 #
 #   git config gpg.ssh.allowedSignersFile allowed_signers
-#   git verify-tag 3.1.5
+#   git verify-tag 3.1.6
 #
 # A good signature covers the tag object, which pins the commit, which pins
 # the whole tree -- so verifying the tag verifies every file at that
 # release. Build a tarball from the verified tag with:
 #
-#   git archive --format=tar.gz --prefix=acme.sh-3.1.5/ 3.1.5 > acme.sh-3.1.5.tar.gz
+#   git archive --format=tar.gz --prefix=acme.sh-3.1.6/ 3.1.6 > acme.sh-3.1.6.tar.gz
 #
 github@neilpang.com namespaces="git" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBTjI0HBJn3uhfT2DsNcFybfAZi3ADbIacMpz1BItKdB