[website] Fix path to README in fetch_documentation.sh

Summary: There is no guarantee the script is run from the website repository root, so the README might belong to a totally unrelated project.

Test Plan:

./scripts/fetch_documentation.sh

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17887