docs: point the concurrent-runs note at the merged library fix

[PrestaEdit]

The README pointed at the fix/checkout-page-closed branch. That fix is now merged into php-library dev (PrestaFlow/php-library#65), which this repo installs as dev-dev, but it is not in a tagged release yet.

🤖 Generated with Claude Code

[github-actions]

PrestaFlow — Test report ✅ · PS 1.7.8.11

18 tests passed in 35s

Suite	✔ Passed	✘ Failed	Duration
Tests\Suites\Checkout	4	0	13s
Tests\Suites\DisplayHome	2	0	0s
Tests\Suites\Regression\NoXssInBlockTitle	4	0	15s
Tests\Suites\Smoke	4	0	0s
Tests\Suites\UpdateTitle	4	0	6s

PS 1.7.8.11

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_{Generated by PrestaFlow/github-action@v2 · commit b7a6420}

[github-actions]

PrestaFlow — Test report ✅ · PS 9.0.0

18 tests passed in 1m 04s

Suite	✔ Passed	✘ Failed	Duration
Tests\Suites\Checkout	4	0	13s
Tests\Suites\DisplayHome	2	0	1s
Tests\Suites\Regression\NoXssInBlockTitle	4	0	21s
Tests\Suites\Smoke	4	0	0s
Tests\Suites\UpdateTitle	4	0	30s

PS 9.0.0

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_{Generated by PrestaFlow/github-action@v2 · commit b7a6420}

[github-actions]

PrestaFlow — Test report ✅ · PS 8.1.7

18 tests passed in 27s

Suite	✔ Passed	✘ Failed	Duration
Tests\Suites\Checkout	4	0	12s
Tests\Suites\DisplayHome	2	0	1s
Tests\Suites\Regression\NoXssInBlockTitle	4	0	8s
Tests\Suites\Smoke	4	0	0s
Tests\Suites\UpdateTitle	4	0	6s

PS 8.1.7

_{Generated by PrestaFlow/github-action@v2 · commit b7a6420}