support/testing/tests/init/test_systemd.py: fix 'on top' typo

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/testing/tests/init/test_systemd.py

@@ -198,7 +198,7 @@ class TestInitSystemSystemdRwFull(InitSystemSystemdBase):
 
 
 # The following tests are all about read-only rootfs, and exercise either
-# using an un-populated factory for /var, or an overlaysfs ontop of a
+# using an un-populated factory for /var, or an overlaysfs on top of a
 # pre-populated /var. They all specialise the TestInitSystemSystemdRo*
 # test cases above.