Changelog: don't pretend Tails 3.4 mitigates Spectre.

See https://www.decadent.org.uk/ben/blog/meltdown-and-spectre-in-debian.html
for details.

debian/changelog

@@ -2,7 +2,7 @@ tails (3.4) unstable; urgency=medium
 
   * Security fixes
     - Install Linux 4.14.0-3 from sid (Closes: #14976). This enables
-      the kernel-side mitigations for Meltdown and Spectre.
+      the kernel-side mitigations for Meltdown.
     - Upgrade curl to 7.52.1-5+deb9u3.
     - Upgrade enigmail to 2:1.9.9-1~deb9u1.
     - Upgrade gimp to 2.8.18-1+deb9u1.