Fix incorrect Wikipedia link for ZX Spectrum

[hayleox]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

The Wikipedia article URL for the ZX Spectrum is wrong; it actually goes to the article about action-adventure games. This PR replaces it with the correct URL.

[ahpook]

Good catch, thanks!