[Docs] Custom filter fix typos

[stlgaits]

No description provided.

[diimpp]

Not sure about this one. While it sounds more natural, do we need to rename file and change entry at table of contents at index.md? And it should be synced with other files like custom_action, custom field type, custom bulk action

WDYT @loic425

[stlgaits]

Fair point (I forgot to change the links in grid/index.md !). In my opinion, they should all be plural or they could all have an article (so 'creating a custom filter' / 'creating a custom action' ...) because it sounds more natural, even though it is correct without, but it's up to you guys :)