Update Telegram link on apps/examples/readme

Summary:
Current link refers to node development group.
The new builders telegram group should be linked here.

Test Plan:
Visit https://github.com/Bitcoin-ABC/bitcoin-abc/blob/master/apps/examples/README.md
and check that the telegram link at the bottom is correctly leading to t.me/eCashBuilders

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16621