add a missed word 'hook' for the description of the post-hook api

We can see it here: http://mongoosejs.com/docs/api.html#schema_Schema-post
This is to compare: http://mongoosejs.com/docs/api.html#schema_Schema-pre

Command used to make an update:
```
grep -ilr "Defines a post for the document" docs | xargs sed -i "" "s/Defines a post for the document/Defines a post hook for the document\./g"
```