fix: replace note to add a link with the actual link

[bryanchriswhite]

Description

Replaces "Superchain (link)" with "Superchain" which links to "./docs/understand/explainer.md" to match the previous link to "the Optimism Superchain".

Tests

N/A: only changes copy

Additional context

I read "(link)" as a note that someone had left for themselves to add a link which apparently never happened.

Metadata

N/A

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🔨 Latest commit	7cb0c2aedbdc91c1adfcbf982a3814cad1362b96

[sbvegan]

Thank you!