Incorrect definition of `ceil` in 13-a-prelude.md

[jordancurve]

13-a-prelude.md says:

You can simply delete this, and remember 1+_ as ceil, cause that removes the need for a word.

But ceil and 1+_ are different. At https://ngn.bitbucket.io/k/#r, ceil 4 returns 4, while 1+_4 returns 5. Transcript:

 ceil:-_-:
 ceil 4
4
 1+_4
5

[razetime]

right, that is only correct for positive integers. I will fix this.