fix(doc-spelling): config spelling_word_list_filename (#5547)

Without this explicit config option, an untracked and unused
`doc/rtd/spelling_wordlist.txt` file is created while running
`tox -e doc-spelling`.

doc/rtd/conf.py

@@ -56,6 +56,7 @@
 templates_path = ["templates"]
 # Uses case-independent spelling matches from doc/rtd/spelling_word_list.txt
 spelling_filters = ["spelling.WordListFilter"]
+spelling_word_list_filename = "spelling_word_list.txt"
 
 # The suffix of source filenames.
 source_suffix = ".rst"