fix directory path for secp256k1 subtree in developer-notes
fix directory path for secp256k1 subtree in developer-notes

Summary: This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/17752 | PR17752]]

Test Plan: Proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8325

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