doc: Run make check instead of unit_tests

Tests in the following file are known to fail
when running the `unit_tests` executable at `test` directory:
test/test-cases/secrules-language-tests/transformations/phpArgsNames.json

Run `make check` at the top directory instead to ignore these failures.
(See https://github.com/owasp-modsecurity/ModSecurity/issues/3328)

README.md

@@ -218,9 +218,7 @@ the utilities, follow the commands listed below:
 ```shell
 $ cd /path/to/your/ModSecurity
 $ git submodule foreach git pull
-$ cd test
-$ ./regression_tests
-$ ./unit_tests
+$ make check
  ```
 
 ### Debugging