Use a script specific description in CLI help

Previous changes used the docstring of the test_generation module,
which does not inform a user about the script.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/scripts/generate_psa_tests.py b/tests/scripts/generate_psa_tests.py
index 81b35c9..c788fd7 100755
--- a/tests/scripts/generate_psa_tests.py
+++ b/tests/scripts/generate_psa_tests.py
@@ -917,4 +917,4 @@
         super().generate_target(name, self.info)
 
 if __name__ == '__main__':
-    test_generation.main(sys.argv[1:], PSATestGenerator)
+    test_generation.main(sys.argv[1:], __doc__, PSATestGenerator)