fix: computed example should not mutate ref value

[skirtles-code]

There are two problems with the existing example:

1. Using count.value++ changes the value of count.value.
2. Despite what it says on the next line, the logged value of plusOne.value will be 1, not 2. Note count.value++ is being used, not ++count.value.

Changing it to count.value + 1 fixes both problems.

[NataliaTepluhina]

Thanks for the fix! 👍🏻