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>

tools/testing/ktest/ktest.pl

@@ -1543,7 +1543,7 @@ sub create_pty {
     my $TIOCGPTN = 0x80045430;
 
     sysopen($ptm, "/dev/ptmx", O_RDWR | O_NONBLOCK) or
-	dodie "Cant open /dev/ptmx";
+	dodie "Can't open /dev/ptmx";
 
     # unlockpt()
     $tmp = pack("i", 0);