Fix PHP version

README.md

 
 Benchmark | PHP | Request | Rq/sec | Score
 --------- | --- | ------- | ------ | -----
-[Hello World](http://www.phpbenchmarks.com/en/benchmark/apache-bench/php-7.1/symfony-4.0.html#benchmark-hello-world) | 7.2 | 1.2 ms | 856 | 19,026
-[Rest API](http://www.phpbenchmarks.com/en/benchmark/apache-bench/php-7.1/symfony-4.0.html#benchmark-rest) | 7.2 | 2.3 ms | 435 | 8,469
+[Hello World](http://www.phpbenchmarks.com/en/benchmark/apache-bench/php-7.2/symfony-4.0.html#benchmark-hello-world) | 7.2 | 1.2 ms | 856 | 19,026
+[Rest API](http://www.phpbenchmarks.com/en/benchmark/apache-bench/php-7.2/symfony-4.0.html#benchmark-rest) | 7.2 | 2.3 ms | 435 | 8,469
 
 Scores are too low ? Do not hesitate to create a pull request, and ask a new benchmark !