std.functional: Fix more DDoc links

[wilzbach]

There's even more broken stuff here here :/

With current CSS styling

@JackStouffer There was dlang/dlang.org#1593, which mainly was about the left-padding here:

```css
table.book tbody a:first-child + a {
    padding-left: .6em;
}
/*style.css:1034 */
table.book tbody a + a {
    padding-right: .6em;
}

Without:

If both left and right-padding are disabled:

[dlang-bot]

Thanks for your pull request, @wilzbach!

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