Fix docstring in sphinxcontrib/spelling/asset.py
Fix docstring in sphinxcontrib/spelling/asset.py

PEP257 compliance

Co-authored-by: Sviatoslav Sydorenko <e7c5f5e12aa12289eaf3aa9899c6f3c862cba97f@sydorenko.org.ua>
sphinx-contrib/spelling,sphinx-contrib/spelling