docs: fix ISR guide link to res.revalidate (#89342)
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What?
Fix a broken link in the ISR guide by switching to a site-absolute
/docs/... URL for the res.revalidate reference.
Why?
Without the leading /, the link can resolve as a relative path and break
depending on where it’s rendered.
How?
Update the ISR guide link target to a site-absolute
/docs/pages/guides/incremental-static-regeneration#on-demand-validation-with-resrevalidate.
Closes NEXT-
Fixes #
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