notmuch: synchronize_flag should be "true"

...and not "True"

According to doc https://notmuchmail.org/manpages/notmuch-config-1/

It also causes a crash in astroid :
https://github.com/astroidmail/astroid/issues/546

modules/programs/notmuch.nix

@@ -89,7 +89,7 @@ in
       extraConfig = mkOption {
         type = types.attrsOf (types.attrsOf types.str);
         default = {
-          maildir = { synchronize_flags = "True"; };
+          maildir = { synchronize_flags = "true"; };
         };
         description = ''
           Options that should be appended to the notmuch configuration file.