sd-json: update comment regarding struct size

The assert at the bottom was updated when the size changed, but
the comment wasn't:

assert_cc(sizeof(sd_json_variant) == 40U);

Follow-up for 337712e777

src/libsystemd/sd-json/sd-json.c

@@ -56,7 +56,7 @@ typedef struct JsonSource {
         char name[];
 } JsonSource;
 
-/* On x86-64 this whole structure should have a size of 6 * 64 bit = 48 bytes */
+/* On x86-64 this whole structure should have a size of 5 * 64 bit = 40 bytes */
 struct sd_json_variant {
         union {
                 /* We either maintain a reference counter for this variant itself, or we are embedded into an