docs - update the example ColumnFilter

[proby-actvo]

The lookups method requires a fourth, run_query, argument.
The get_filtered_query method requires a fourth, model, argument.

Also updated the docs, a minor clarification, that the above two are methods not fields.

It also looks like my editor removed some extra whitespace on save, I hope that's ok.

I'm also willing, if there's interest, to update the column filter code to add a BaseFilter(ABC) base class for all filters. Something to help enforce API consistency. Roughly:

class BaseFilter(ABC):
    """
    Base class for all filters.

    All column filters should inherit this class and implement the required methods.
    """
    @abstractmethod
    def lookups(self, request: Request, model: Any, run_query: Callable[[Select], Any]) -> List[Tuple[str, str]]:
        raise NotImplementedError

    @abstractmethod
    def get_filtered_query(self, query: Select, value: Any, model: Any) -> Select:
        raise NotImplementedError

[birddevelper]

Hi @proby-actvo , Thanks for your contribution. I think this version can be more clear:

Suggested change

	A ColumnFilter is a class that defines a filter for a column. A few standard filters are implemented in `sqladmin.filters` module. Here is an example of a generic ColumnFilter. Note that the fields `title` & `parameter_name` and the methods `lookups` & `get_filtered_query` are required.
	A ColumnFilter is a class that defines a filter for a column. A few standard filters are implemented in `sqladmin.filters` module. Here is an example of a generic ColumnFilter. Note that the fields `title` and `parameter_name`, and the methods `lookups` and `get_filtered_query` are all required in a filter class.

[proby-actvo]

Thanks @birddevelper -- good suggestions, much appreciated. I've adopted your suggestions.