fix(www/dark-mode-toggle): title and aria-label[object Object] issue

[armujahid]

Description

This fixes incorrect title and aria-label issue of dark mode toggle button

Related: #21633

Note that I haven't tested this fix locally yet because gatsby develop in www is taking too much time on my PC.

[armujahid]

I have tested this and it's working fine.

[LekoArts]

Thanks! 👍

[gatsbot]

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