[tg bot] conditionally render tx or txs depending on txcount

Summary:
T2972
Depends on D13282

If a block only has one tx, don't say "1 txs," say "1 tx."

Test Plan:
npm test
npm run msgs and note msgs with 1 etoken tx or 1 tx do not say txs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13283