singularize variable name on add/remove methods for EntityGenerator

Not a big change but corrects an error of plural.

Previously :
```php
    public function addComment(EntityGeneratorComment $comments)
    {
        $this->comments[] = $comments;

        return $this;
    }
```

With this PR :
```php
    public function addComment(EntityGeneratorComment $comment)
    {
        $this->comments[] = $comment;

        return $this;
    }
```