DOC Use pipelines module name instead of pipieline following default project files.

The command `startproject` creates, among other files, the file `pipelines.py` where new users create their pipelines. Currently there are two examples of pielines wich refers to the module `pipeline` instead of `pipelines` and this might confuse new python/scrapy users.

This pull request change the module name `pipeline` to `pipelines` in the user examples.