fix: add 's' suffix to _processMetaSub following previous change

[dilshansdoq]

Hi, This commit is regarding a missing rename in tinode.js file following the change made in topic.js (commit: https://github.com/tinode/tinode-js/commit/ef312f9d1b030f577d9ff4d5047d24af2fc50383). Original Bug was reported on google groups https://groups.google.com/g/tinode/c/ghm7jknaYuY

[or-else]

Thanks!