Fix url for PDB sequence metadata download in `PDBManager`

[a-r-j]

Reference Issues/PRs

What does this implement/fix? Explain your changes

What testing did you do to verify the changes in this PR?

Pull Request Checklist

• [ ] Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• [ ] Added appropriate unit test functions in the ./graphein/tests/* directories (if applicable)
• [ ] Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• [ ] Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/protein/test_graphs.py)
• [x] Checked for style issues by running black . and isort .

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