twister: Fix --disable-coverage-aggregation help text

There is no flag named --coverage-split, switch help to
--coverage-per-instance

Signed-off-by: Jeremy Bettis <[email protected]>

Author: jeremybettis

scripts/pylib/twister/twisterlib/environment.py

@@ -400,8 +400,8 @@ def add_parse_arguments(parser = None) -> argparse.ArgumentParser:
                 action="store_true", default=False,
                 help="""Don't aggregate coverage report statistics for all the test suites
                         selected to run with enabled coverage. Requires another reporting mode to be
-                        active (`--coverage-split`) to have at least one of these reporting modes.
-                        Default: %(default)s""")
+                        active (`--coverage-per-instance`) to have at least one of these reporting
+                        modes. Default: %(default)s""")
 
     parser.add_argument(
         "--test-config",