docs: Correct description for getblockstats's txs field

Summary:
It does count the coinbase transaction.

Refs #19766

This is a backport of core#19777

Test Plan:
Verified the output for recent blocks on a block explorer.

$ src/bitcoin-cli getblockstats '"000000000000000009078f13b271c8bd3075bf77ed1fb70b08ac0cbc92545ce2"'
{
  ...
  "txs": 14,
  ...
}
$ src/bitcoin-cli getblockstats 704983
{
  ...
  "txs": 9,
  ...
}

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10121