Fix documentation about PLUGIN_PATH setting

`PLUGIN_PATH` setting has been replaced by `PLUGIN_PATHS` and it should be a list.

```
% make ssh_upload                             
pelican /home/yoyo/projects/pelican-website/content -o /home/yoyo/projects/pelican-website/output -s /home/yoyo/projects/pelican-website/publishconf.py 
WARNING: PLUGIN_PATH setting has been replaced by PLUGIN_PATHS, moving it to the new setting name.
WARNING: Defining PLUGIN_PATHS setting as string has been deprecated (should be a list)
```