doc/dependencies: Fix typo libsrvg->librsvg

Summary: This is a backport of Core PR15684

Test Plan: grep -r libsrvg *

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8214