tmux: fix hash, looks like the upstream tag was moved

--- a/configure.ac 1970-01-01 01:00:01.000000000 +0100
+++ b/configure.ac 1970-01-01 01:00:01.000000000 +0100
@@ -1,6 +1,6 @@
# configure.ac

-AC_INIT(tmux, 2.5-rc)
+AC_INIT(tmux, 2.5)
AC_PREREQ([2.60])

AC_CONFIG_AUX_DIR(etc)

pkgs/tools/misc/tmux/default.nix

···
     owner = "tmux";
     repo = "tmux";
     rev = version;
-    sha256 = "0h0c6rwp4bjifqx9wpx7y0bmayvrvvgh9211rdk4nsf81j17n6vx";
+    sha256 = "0zwjngfaqrlwwbzicc5pq5pyws8f1qghcajvj0hwkipj51hqyswf";
   };
 
   nativeBuildInputs = [ pkgconfig autoreconfHook ];