docs: fix incorrect quotes in assertTableColumnExists argument

[CodeWithDennis]

No description provided.

[saade]

Hey,

Pull requests to fix/ improve documentation should be opened in the filament/filament repository where the source files for the docs exists. You've edited the compiled html.

This is the file you need to edit in order to update the documentation properly.
https://github.com/filamentphp/filament/blob/3.x/packages/tables/docs/12-testing.md

[CodeWithDennis]

Woops, will do.