docs: fix malformed SubQuery Academy link (missing .network)

[boleklebovski]

Problem

docs/build/integrations/indexers/subquery.md:15 links to https://academy.subquery — the .network TLD is missing, so the hostname does not resolve to the docs site (it returns 502).

Every other SubQuery link in the very same file uses the full host, including line 357 which points at https://academy.subquery.network/, so this is clearly a typo rather than an intentional shorthand.

It is also the "To find out more, visit our documentation" link in the intro paragraph — the first thing a reader clicks on that page.

Fix

https://academy.subquery → https://academy.subquery.network/

Verification

Broken URL returns 502 on two separate checks; the replacement returns 200 on two separate checks.

Worth noting for the link-checking setup: the automated checker does not flag this one because the host answers with 502 rather than 404, so a status-code filter that only looks for 404 will keep missing malformed hostnames like this.