ktest.pl: Fix spelling mistake "Cant" -> "Can't"

There is a spelling mistake in an error message. Fix it.

Link: https://lkml.kernel.org/r/20200810100750.61475-1-colin.king@canonical.com



Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>