Fix category icons

@marcia @luke The category icons showed up as black and not centered from !14855 (merged)

I hope it's ok that I took the initiative to fix this.

I took the format of the other category SVGs and copied it over. It looks fine on my local with these changes.

Here's exactly the change

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 76 76" enable-background="new 0 0 76 76"><g fill="#231f20">

New version (taken from other SVGs)

<svg viewBox="0 0 76 76" width="76" height="76" xmlns="http://www.w3.org/2000/svg">;

(and removed the closing </g> tags)