Fixed link to BC5CDR dataset (Closes #909)

[drAbreu]

• Dataset: BC5CDR
• Description of the problem: The links to the bc5cdr dataset are no longer valid.
• Data: official, mirror

Solution I change the links of the data to this mirrored repository since the new domain of BioCreative seems to still have problems.

Checkbox

• [x] Confirm that this PR is linked to the dataset issue.
• [x] Create the dataloader script hub/hub_repos/my_dataset/my_dataset.py (please use only lowercase and underscore for dataset naming).
• [x] Provide values for the _CITATION, _DATASETNAME, _DESCRIPTION, _HOMEPAGE, _LICENSE, _URLs, _SUPPORTED_TASKS, _SOURCE_VERSION, and _BIGBIO_VERSION variables.
• [x] Implement _info(), _split_generators() and _generate_examples() in dataloader script.
• [x] Make sure that the BUILDER_CONFIGS class attribute is a list with at least one BigBioConfig for the source schema and one for a bigbio schema.
• [x] Confirm dataloader script works with datasets.load_dataset function.
• [x] Confirm that your dataloader script passes the test suite run with python -m tests.test_bigbio_hub <dataset_name> [--data_dir /path/to/local/data] --test_local.

[dinhngoc267]

Hi, @drAbreu Do you have link to the MeSH dictionary which use for this dataset? I have found in another repo but the bc5cdr has some mention ids which don't exist in the MeSH dictionary, I need to find the right version.