Find all files with the extension ".txt" and execute the command "sed 's/adjective/\L&/g' {} \;" on each of them. The command "sed 's/adjective/\L&/g' {} \;" will replace all occurrences of the word "adjective" with the same word in lowercase.
find . -name "*.txt" -exec sed 's/adjective/\L&/g' {} \;