update concurrency part in README

[zmiklank]

${{ github.head_ref }} is not available for issue_comment target, use simple string as an exemplary concurrency group. Use tfaga@v2 for this, as this is the first version supporting concurrency mechanisms.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.28%. Comparing base (6d060a1) to head (2d5e1ec). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #133 +/- ## ========================================== - Coverage 99.29% 99.28% -0.01% ========================================== Files 8 8 Lines 564 563 -1 Branches 61 61 ========================================== - Hits 560 559 -1 Misses 4 4 ```

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[jamacku]

@zmiklank maybe also update the second example with v2

[zmiklank]

[test-all]