pdf_path cannot be without a trailing slash

[HiromuHota]

Describe the bug

docs_path and pdf_path are typically followed by a slash "/" like below:

docs_path = "tests/data/html/"
pdf_path = "tests/data/pdf/"

doc_preprocessor = HTMLDocPreprocessor(docs_path, max_docs=max_docs)

corpus_parser = Parser(
    session,
    parallelism=PARALLEL,
    structural=True,
    lingual=True,
    visual=True,
    pdf_path=pdf_path,
)

While docs_path can be without a trailing slash, pdf_path cannot.

To Reproduce
Steps to reproduce the behavior:

1. Do not set a trailing slash like pdf_path = "tests/data/pdf"
2. Parse

Expected behavior

pdf_path without a trailing slash works too.

Error Logs/Screenshots
If applicable, add error logs or screenshots to help explain your problem.

I/O Error: Couldn't open file 'tests/data/pdf112823.PDF': No such file or directory.

Environment (please complete the following information):

• OS: macOS 10.15.4
• PostgreSQL Version: N/A
• Poppler Utils Version: N/A
• Fonduer Version: 0.8.2