dm vdo: Fix spelling mistake "postive" -> "positive"

There is a spelling mistake in a vdo_log_error message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Author: ColinIanKing

drivers/md/dm-vdo/dm-vdo-target.c

@@ -412,7 +412,7 @@ static int __must_check parse_memory(const char *memory_str,
 
 		result = kstrtouint(memory_str, 10, &value);
 		if (result) {
-			vdo_log_error("optional parameter error: invalid memory size, must be a postive integer");
+			vdo_log_error("optional parameter error: invalid memory size, must be a positive integer");
 			return -EINVAL;
 		}